Learn Portuguese (Portugal): Ele fala mal inglês.

In Portuguese, adverbs that describe a verb usually go right after the verb they modify. So, fala mal stays together, pushing the object (inglês) to the end of the sentence.

Would mal change to a feminine form if the sentence started with Ela (She)?

No! Just like muito and pouco when they act as adverbs, mal is invariable. This means it never changes its spelling to match the subject's gender or number, because it describes the verb (fala) and not the person.

What is the difference between mal and mau?

mal is an adverb meaning 'badly' or 'poorly' — it describes an action (like speaking). mau is an adjective meaning 'bad' — it describes a noun (like a bad dog). Because we are talking about how he speaks, we must use mal.

Why is inglês written with a lowercase 'i'?

Unlike in English, words for languages, nationalities, and days of the week are written entirely in lowercase in Portuguese unless they appear at the very beginning of a sentence.