Sind die Kommas in “ein hübsches, schlankes, blondes, deutsches Mädchen” notwendig?
BACKGROUND: In English, it is possible to place several QUALIFYING adjectives before a noun, as in “a beautiful slim blonde German girl”, BUT no commas are needed (or, indeed, possible) between them. That is typical of Germanic languages, in opposition to Romance ones like Spanish, where such sequences are impossible, cf. “*una preciosa esbelta rubia alemana chica”. In Germanic, so the standard explanation goes, nouns need not move leftwards, and qualifying adjectives remain before them = …[A + A + A + N]…, whereas in Romance nouns must generally move leftwards, and series of qualifying adjectives must follow them = …[N+A]+A]+A]… So far, so good. What complicates the picture is that all Romance post-nominal adjectives BUT THE FIRST MUST be separated by commas and/or conjunctions (cf. “una chica ALEMANA, rubia, esbelta, (y) preciosa” vs. “*una chica alemana rubia esbelta preciosa”. This indicates that Romance post-nominal adjectives are, in fact, reduced clauses with invisible subjects co-referring with “chica”. A problem, then, arises in German: if the commas ARE needed in “ein hübsches, schlankes, blondes, deutsches Mädchen”, such adjectives must also at bottom be elliptical clauses with hidden subjects co-referring with “Mädchen” (so the “–es” ending indicates), BUT, if so, the adjectives should have PREDICATIVE form, with NO “–es” ending (cf. “Das Mädchen ist hübsch/*hübsches”), and we should expect “*ein hübsch, schlank, blond, deutschES Mädchen” (note that the adjective closest to the noun needs no comma, as in Spanish), but that is NOT what native speakers do. Summarizing: 1) if the commas ARE needed, then the use of “hübsches”, “schlankes”, etc., rather than “hübsch”, “schlank”,… is unexpected; 2) if the commas are NOT POSSIBLE, German is like English and there is no problem; and 3) if the commas are POSSIBLE, but NOT NECESSARY, then the construction is a theoretical mess requiring urgent clarification, :-).
QUESTION (FOR NATIVE SPEAKERS ONLY!): ARE THE COMMAS NEEDED OR NOT? [Please note that the question concerns ONLY QUALIFYING adjectives; adjectives with deictic, modal or quantificational content must precede qualifying adjectives WITHOUT commas; ignore them here].
Addenda. I'm sorry, this little italki window made me simplify too much in my first sentence and also in point 3): of course, commas/pauses are possible almost everywhere, in all languages, presumably,...., and even between the most tightly-related syntactic elements (e.g., verb and its object, preposition and its object) when speakers want to mark contrasts with respect to preceding discourse (cf., "It is NÉAR, London, but not ÍN, London"; "I did SEE, her but didn't actually TALK, to her", etc.) and when they want to create special stylistic effects (e.g., an emphatic rendition of each of the attributes). The possibility of making such pauses for such reasons is, therefore, irrelevant here. The real issue is whether the comma/pause is OBLIGATORY or not, and the interesting fact, it seems to me, is that the adjective closest to the noun, apparently, need not be followed by a comma/pause, whereas the others (perhaps) MUST be. Correspondingly, the closest adjective following a noun in Spanish (etc.) CANNOT BE PRECEDED by a comma/pause, whereas the following ones may (but need not) be. The reason why that quirk is NOT problematic in Spanish (etc.) is that in Spanish (etc.) predicative adjectives must AGREE with their noun subjects in number and gender, just as attributive ones, and they do [so, OK]; in English, where nouns and adjectives do not visibly agree in ANY case, the possible difficulty cannot arise [so OK, too]; but IN GERMAN, predicative adjectives do NOT agree with their subjects; only attributive ones agree (in gender, number, case) with their head nouns; hence, those adjectives in "ein hübsches, schlankes, .... Mädchen" must be "attributive". But, if so, why does the adjective closest to the noun ("deutsches", in the example) NOT need a comma, whereas the preceding ones MUST be followed by a comma (if they, indeed, must)? Of course, when I said in 3)that the construction is a theoretical mess and should be clarified, I did not mean there is anything "wrong" with it; languages are always "right"; the "mess" is only for us grammarians who try to build elegant theories of a marvellously complex object, :-). Oh, and, pace yujin, in English it is NOT necessary to insert comma/pause between the adjectives in "a beautiful blond girl", although you MAY insert it if you want to create special effects, i.e., for the irrelevant reasons already mentioned.

Question

Sind die Kommas in “ein hübsches, schlankes, blondes, deutsches Mädchen” notwendig?
BACKGROUND: In English, it is possible to place several QUALIFYING adjectives before a noun, as in “a beautiful slim blonde German girl”, BUT no commas are needed (or, indeed, possible) between them. That is typical of Germanic languages, in opposition to Romance ones like Spanish, where such sequences are impossible, cf. “*una preciosa esbelta rubia alemana chica”. In Germanic, so the standard explanation goes, nouns need not move leftwards, and qualifying adjectives remain before them = …[A + A + A + N]…, whereas in Romance nouns must generally move leftwards, and series of qualifying adjectives must follow them = …[N+A]+A]+A]… So far, so good.  What complicates the picture is that all Romance post-nominal adjectives BUT THE FIRST MUST be separated by commas and/or conjunctions (cf. “una chica ALEMANA, rubia, esbelta, (y) preciosa” vs. “*una chica alemana rubia esbelta preciosa”. This indicates that Romance post-nominal adjectives are, in fact, reduced clauses with invisible subjects co-referring with “chica”. A problem, then, arises in German: if the commas ARE needed in “ein hübsches, schlankes, blondes, deutsches Mädchen”, such adjectives must also at bottom be elliptical clauses with hidden subjects co-referring with “Mädchen” (so the “–es” ending indicates), BUT, if so, the adjectives should have PREDICATIVE form, with NO “–es” ending (cf. “Das Mädchen ist hübsch/*hübsches”), and we should expect “*ein hübsch, schlank, blond, deutschES Mädchen” (note that the adjective closest to the noun needs no comma, as in Spanish), but that is NOT what native speakers do. Summarizing: 1) if the commas ARE needed, then the use of “hübsches”, “schlankes”, etc., rather than “hübsch”, “schlank”,… is unexpected; 2) if the commas are NOT POSSIBLE, German is like English and there is no problem; and 3) if the commas are POSSIBLE, but NOT NECESSARY, then the construction is a theoretical mess requiring urgent clarification, :-). 
QUESTION (FOR NATIVE SPEAKERS ONLY!): ARE THE COMMAS NEEDED OR NOT? [Please note that the question concerns ONLY QUALIFYING adjectives; adjectives with deictic, modal or quantificational content must precede qualifying adjectives WITHOUT commas; ignore them here].
Addenda. I'm sorry, this little italki window made me simplify too much in my first sentence and also in point 3): of course, commas/pauses are possible almost everywhere, in all languages, presumably,...., and even between the most tightly-related syntactic elements (e.g., verb and its object, preposition and its object) when speakers want to mark contrasts with respect to preceding discourse (cf., "It is NÉAR, London, but not ÍN, London"; "I did SEE, her but didn't actually TALK, to her", etc.) and when they want to create special stylistic effects (e.g., an emphatic rendition of each of the attributes). The possibility of making such pauses for such reasons is, therefore, irrelevant here. The real issue is whether the comma/pause is OBLIGATORY or not, and the interesting fact, it seems to me, is that the adjective closest to the noun, apparently, need not be followed by a comma/pause, whereas the others (perhaps) MUST be. Correspondingly, the closest adjective following a noun in Spanish (etc.) CANNOT BE PRECEDED by a comma/pause, whereas the following ones may (but need not) be. The reason why that quirk is NOT problematic in Spanish (etc.) is that in Spanish (etc.) predicative adjectives must AGREE with their noun subjects in number and gender, just as attributive ones, and they do [so, OK]; in English, where nouns and adjectives do not visibly agree in ANY case, the possible difficulty cannot arise [so OK, too]; but IN GERMAN, predicative adjectives do NOT agree with their subjects; only attributive ones agree (in gender, number, case) with their head nouns; hence, those adjectives in "ein hübsches, schlankes, .... Mädchen" must be "attributive". But, if so, why does the adjective closest to the noun ("deutsches", in the example) NOT need a comma, whereas the preceding ones MUST be followed by a comma (if they, indeed, must)? Of course, when I said  in 3)that the construction is a theoretical mess and should be clarified, I did not mean there is anything "wrong" with it; languages are always "right"; the "mess" is only for us grammarians who try to build elegant theories of a marvellously complex object, :-). Oh, and, pace yujin, in English it is NOT necessary to insert comma/pause between the adjectives in "a beautiful blond girl", although you MAY insert it if you want to create special effects, i.e., for the irrelevant reasons already mentioned.

Answer

To add on to what Daniel said,

You DO need commas in English.

“a beautiful slim blonde German girl” is NOT grammatically correct

This is grammatically correct:

"A beautiful, slim, blonde German girl"

You can test this by  using "and" (similar to what Daniel said about german grammar!)

You can usually put "and" in between the last adjective and the noun

So.. "A beautiful, slim, and blonde German girl"

Answer

Hi Luis,

luckily I have informed myself about that topic lately. ;) Oh my, that was you, right? ;)

if you have a noun with multiple adjectives, the commas are needed, if the adjectives are coequal.

"ein hübsches, schlankes, blondes, deutsches Mädchen."

Basically you can determine that with a test of substitution. You just have a look, if you can put a "und" in between the different adjectives. If you can put it, then they are coequal on the semantic level and a comma is needed. If an "und" makes no sense, then you don't put a comma, e.g. "allgemeine soziale Situation". Here, you can -not- say "allgemeine und soziale Lage", cause it makes no sense - the "allgemein" refers to the "social", hence there is no comma, but it gets declinated after the noun. It's different of course with modal or quantificional adjectives:

"ein hübsches, außerordentlich schlankes, blondes, deutsches Mädchen." No declination there...

So, in your sentence, you need the commas, cause the adjectives are sematically coequal. Still, they all have the ending referring to the noun. When they are equal it's basically just an enumeration. Every single adjective could stand alone.

So possibility 1) is correct. Well, it's unexpected for you, but goes perfectly after the German rules! ;)

Lesson: German is not English and not Spanish... and it's weird :P

Positive about that is, that you can point out the special meaning of a sentence by using or not using commas. An example from Wikipedia:

1. Bei der Produktion werden neue, umweltfreundliche Verfahren angewandt.
2. Bei der Produktion werden neue umweltfreundliche Verfahren angewandt.

For what they both mean and additional information, have a look at that chapter (German though):
http://de.wikipedia.org/wiki/Kommaregeln#Reihungen_von_Adjektiven

Regards,
Daniel

Answer

An attributive adjective and a noun don't need a comma.
A listing or enumeration of coequal, multiple attributive adjectives in front of a noun however does. But of course not the last piece of the chain. When you want to see the enumeration as one semantic unity and therefore see it as a united attributive adjective, simply use the sentence above again.

A N (no comma)
A, A, A = A* (comma as link; not after the last, not before the first)
A* N (semantic; no comma)
A, A, A N (commas as links of the enumeration of coequal attributive adjectives)

Of course, I can only interpret the rules I've read, for more you have to ask a linguist. 
And I found the rule of multiple coordinate adjectives for english, and that was what Yujin meant here, I guess. Since I don't own an digital english grammar, let me quote Wikipedia (http://en.wikipedia.org/wiki/Comma) here:

"Between adjectives
A comma is used to separate coordinate adjectives; that is, adjectives that directly and equally modify the following noun. Adjectives are considered coordinate if the meaning would be the same if their order were reversed or if and were placed between them. For example:
The dull, incessant droning but the cute little cottage.
The devious lazy red frog suggests there are lazy red frogs (one of which is devious), while the devious, lazy red frog does not carry this connotation."

If those rules are -may- or -must-, is another thing. Also in German: It is highly probable, that the people will understand you, even without the commas. In everyday writing, e.g. facebook and so, most of the people don't use them. But of course that language mustn't be mixed with the standard language.

I personally do put the comma and point out pauses of tension with a dash.

Daniel

Answer

I can just answer to this additional point:
"But, if so, why does the adjective closest to the noun ("deutsches", in the example) NOT need a comma, whereas the preceding ones MUST be followed by a comma (if they, indeed, must)?"

For all other points and thoughts, I'm as a non-linguist unfortunately qualified.

And also on this one, I can only tell you, what the rule is, and what the theory around it could be IMHO.

I the reason for commas being there is simply found in the enumeration of several attributive adjectives. 
One doesn't have or need a comma, of course, since it relates directly to its noun, which is located next to it.

"ein deutsches Mädchen"

However, adding another coequal attributive adjective, and not using an "und" gives us the comma - maybe just like in normal enumerations. That doesn't have to do anything yet with pointing out important things like in speech.

to be continued, due to 2000 sign limit.

Answer

Furthermore, now I tried to compare the rules to the english ones. Are you positive on english not needing the commas between adjectives? In all sites or grammar books that are covering that topic, I found the rule:

Use commas to separate two or more coordinate adjectives that describe the same noun. Be sure never to add an extra comma between the final adjective and the noun itself or to use commas with non-coordinate adjectives.

To determine, whether you need it or not, you should try to say the adjectives in reverse order. If it still has sense, you should put commas (for coordinate adjectives). At least they say so.

And I also thought to remember from my english classes, that the formal english requires adjectives in that case. In French however I know, that they don't use many commas in that case, but more express it with "et".

Hope, we can find out, what's correct. Maybe at the end, German and English are not so far away from each other - at least by the formal rules. :)

Daniel