That's it, in a nutshell. If you are linking two nouns (or a noun and a pronoun), use the copula, if you are using a noun and an adjective, you use the verb bí (which is tá in the present tense).
Is fear é - "He is a man"
Tá sé fuar - "He is cold"
There is more to the copula than that, but that's the best place to start off.
No, because "bilingual" is a predicative adjective in "you are bilingual" (it comes after the verb in English).
Only attributive adjectives agree with their nouns, predicative adjectives don't change, so "the bilingual people are talking" is Tá na daoine dátheangacha ag caint whereas "the people are bilingual" is Tá na daoine dátheangach.