The handiest of all is to just download the keyboard you want. If you're using Windows go to control panel ---> change keyboards or other input methods(Under clock, language and region)  ---> add ---> then choose the language you want. You can toggle between your default keyboard and any you've added by pressing SHIFT + ALT.

Yes, and please don't replace ß by β (the Greek letter beta). Those letters look quite similar, but are not the same.

Here are some examples for words where the "Umlaute" totally change the meaning:

Vogel (bird) vs. Vögel (birds) [there are plenty of examples where the Umlaute change a word from singular to plural, Nagel/Nägel, Boden/Böden]

schwul (gay) vs. schwül (humid)

Bar (bar) vs. Bär (bear)

musste (had to) vs. müsste (ought to)

If you only have an English keyboard and want to write in a foreign language then all you have to do is learn the codes for the letters you're interested in.

So for Spanish the ones you're interested in are á, é, í, ó, ú, ñ and ¡ and ¿. It took me about 10 minutes to learn, it's really not hard. I'm not learning German but they are just as easy to learn e.g. ü or ß. 

http://sites.psu.edu/symbolcodes/windows/codealt/

Edit: Same but if you have a Mac

http://sites.psu.edu/symbolcodes/mac/codemac/

Very good point to make, thank you, Susanne!

Regarding the ß I just wanted to point out that in Swiss German ß is not used at all, so spelling ss is a perfectly correct solution and definitely much better than using a capital B instead - which makes no sense at all, but some people do it.

I use the respective keyboard layouts for the target language.

In the Swedish layout, I have no problem with not seeing the few letter keys that are different. I can memorise them. Only with non-letter characters it's sometimes annoying.

With the Persian layout it's a bit more difficult. I have now mail-ordered keyboard stickers to glue onto my keys. They are transparent plastic with yellow persian characters and I can still see the white latin letters below.