Berber names a person by the people they belong to, so it behaves like Canadian or Thai: it takes a capital letter, and you can count it — one Berber, three Berbers. The same spelling also names the language these people speak, so the sentence around the word tells you which one you are reading. One fact travels with this word: the source marks it as possibly offensive to some people, and it comes from the same old root as barbarian, which once meant only 'a foreigner whose speech we cannot follow'. Because of that history, writers often check what a group calls itself before picking a name for it.

When Berber names a language it stops being countable: you say speak Berber, learn Berber, in Berber, with no a and no -s. Notice what the sources together tell you here. The learner dictionary says 'one of the languages', and the other source says 'a group of closely related languages' — so Berber is less like one fixed language and more like a family of them, and two speakers from different areas may not understand each other easily. The capital letter stays, the way it does for Thai or Arabic.