Learn Portuguese (Portugal): O taxista fala inglês.

Words ending in -ista for professions (like taxista or dentista) don't change their spelling based on gender. Instead, the article tells you who it is. O taxista is a male taxi driver, while A taxista would be a female taxi driver.

Why isn't the word for English (inglês) capitalized here?

Unlike in English, languages and nationalities are not capitalized in Portuguese unless they start a sentence. So, inglês is written with a lowercase letter.

Could I use fala to say "I speak English"?

No. The verb falar (to speak) changes depending on who is speaking. Fala is the form for he, she, or formal you. To say "I speak," you would use falo.

Why is there no word for "the" before inglês?

Directly after the verb falar (to speak), Portuguese generally does not use a definite article (like o or a) before the name of the language. You just say the language directly.