Can anybody explain to me why the A in “age” is not pronounced like the A in “language”?
That’s the way it is in English, Jorge! Each vowel can have different sounds. The ‘a’ in ‘age’, is pronounced [ei]; in the first syllable of ‘language’, it’s [a]; in the second (unstressed) syllable, it’s [i]. Other sounds for the ‘a’ are [a:], as in ‘car’,  (schwa), as in ‘ago’, [e], as in ‘any’ and [o:], as in ‘also’.
Welcome and enjoy!
Part of the reason, Jorge, is that the /ei/ in age is in a stressed syllable, while in language the vowel is unstressed.
So, Mister Micawber, is this a rule? And can I say that this rule applies to “chocolate” and “classmate”?
Pronunciation does not have ‘rules’ in the sense that grammar does, Jorge, but–yes, unstressed vowel sounds tend to be shortened and centralized. Your second pair of words seems to reflect this in the same way that your first pair does.
Be cautioned, however, that there are other factors also-- for instance, the words or wordparts may have very different etymologies.
If I may just add a quick footnote: while the ‘a’ in ‘chocolate’ is a short unstressed sound , the second ‘a’ in ‘classmate’ is pronounced [ei], although it’s in the unstressed syllable.