"He speaks English."
A quick google (not scientific I know!) gives probably more usage with lower-case than capitalised. For example: https://www.zougla.gr/automoto/paraxena/diafora/article/an-kseris-na-milas-aglika-pas-pio-grigora-stin-kavala .
Απίστευτο! Αν ξέρεις να μιλάς αγγλικά πας πιο γρήγορα στην Καβάλα...
Would you say this usage is non-standard or not generally accepted though?
μιλάει" is he/she/it speaks. "μιλώ" and "ομιλώ" are two ways to say I speak.
See this conjugation page for each of them
and check the comments at the bottom of this page.
"Αυτός μιλάει αγγλικά." is correct and accepted. If your sentence was not accepted there was a mistake. However, by giving one word from a sentence you can't expect us to understand where your problem was since we are unable to see your sentence. You should have made a REPORT
Here's how to REPORT.
Go to the bottom of the exercise page where you'll see "REPORT" Click on that:
Then choose what you want to REPORT: (You'll see these to choose from.)
-The audio does not sound correct-
-The dictionary hints on hover are wrong.
The audio is missing.
-The dictionary hints on hover are missing.
-My answer should be accepted.
-Something else went wrong.
If you don't understand why your sentence was not accepted comment here after you have made the REPORT.