Learn Portuguese (Portugal): Os alemães falam inglês?

The singular word for a German male or the German language is alemão. To make words ending in -ão plural, the ending changes to -ões, -ãos, or -ães. For alemão, the plural form is alemães.

Why aren't alemães and inglês capitalized in this sentence?

Unlike in English, words for nationalities (like alemães) and languages (like inglês) are written with lowercase letters in Portuguese.

Why does the verb falam end in an m?

falam is the they form of the verb falar (to speak). Because the subject of the sentence is plural (the Germans or Os alemães), we need the matching plural verb form, which ends in -m.

English says Do Germans speak..., but this sentence uses Os. Do I need it?

Yes. In Portuguese, when you talk about a whole group of people in a general sense, you usually need the definite article (in this case, os for masculine plural). So, Germans translates to os alemães (literally, the Germans).