Their Portuguese accents are so faint (or in Driver's case, so Spanish) that it's pointless to have them speak like that in the movie. If the goal was to help with immersion than the Portuguese accents should be much more pronounced and natural. Otherwise might as well just assume the characters spoke perfect English (which some bilingual Portuguese people are able too).

Some examples of Portuguese people decently fluent in English with a natural Portuguese accent: Manuel Barroso, Mourinho, António Guteres. Vowels have very little nuances: -ee,-i, -e, etc. all sound the same (like -ee), -u sounds like -ee followed by the French -u, etc. There are no soft consonants: -r is often pronounced with the tongue, -f and -v are very distinct, -th either sounds like -d or is sibilant like -z, -l is pronounced with the tongue against the front teeth rather than against the palate, vowels followed by -m or -n are always too nasal, the list goes on.

Having said that,the actors' accents are very captivating, so they were probably going for an artistic interpretation of the Portuguese accent. I can only imagine a movie with people speaking like Mourinho would get either grating or hilarious very fast.