Simple sentence structure in Chinese

there is no plural form on the object 这是老师不是老师们 the plural is only used for the subject 我 is modified into 我们 right?

Yeah, you are right. In Chinese the plural of noun is not necessary all the time.

Can you explain the rules when it is necessary and when not? As far as I know it is not necessary, when there is already a counting number set before. Like in 这是两个人。

The plural form of nouns Usually the singular and plural forms of a noun are identical and the plural is expressed in three ways: 1By premodifying numeral-measure words or other words implying the plural. eg三(sān)支(zhī)铅(qiān)笔(bǐ)[Three pencils]，十(shí)七(qí)个(ɡè)孩(hái)子(zi)[seventeen children]，很(hěn)多(duō)书(shū)[many books]，这(zhè)些(xiē)书(shū)[these books] 2By other elements in the sentence. eg客(kè)人(rén)都(dōu)来(lái)了(le)。[All guests have arrived]东(dōnɡ)西(xi)全(quán)在(zài)桌(zhuō)子(zi)上(shànɡ)。[All things are on the table] 3By suffixing们(men)to the noun.And 们(men)can just be used to after a noun which indicate people. eg 朋(pénɡ)友(you)们(men)[friends] By suffixing们(men),which is pronounced in the neutral tone, to the noun. When thus used, no other words expressing the plural number can be used. 五(wǔ)个(ɡè)朋(pénɡ)友(you)[five friends] we can't say五(wǔ)个(ɡè)朋(pénɡ)友(you)们(men)

okay, so if I got it right I always can only use one of the plural forms. If I do mention the quantity I don't use the 们 anymore. Can I say for example: 德国人们都说德语 or does the 们 I use at the beginning means that I can't use the 都 anymore? I think so?! That would mean I can choose between: 法国人们说法语 中国人都说汉语 Right???

When you want to emphersize the plural you can use 们, otherwise it is no need. In your sentences法国人说法语.It's no need to emphasize plural. 我的朋友说法语。（in this sentence, it's not clear that I talk about one friend or many friends.) 我的朋友们说法语。(This sentence shows I have more than one friend speaking French.) 我的朋友都说法语。(This sentence has the same effect with the second.)

都is an adverb. So it can be used after 们. They can use in the same sentence.

Why I do need to use the sign 们 in the sentence 法国人说法语 ? isn't there a difference between 法国人们and 一个法国人？

You don't need put 们after 法国人. The correct one should be 法国人说法语。It's enough.