Definitely. Their English even after 30+ years in America is at the level where our English-English conversations are probably at the elementary grade level (except with a few more proper nouns). I used to think that they were at least better in understanding
my natural English conversation, but I've realized that I reflexively shift to using much simpler sentences when speaking to them. But when they're with Vietnamese friends or family, they have conversations (in Vietnamese) just as normal adults typically do in their native language.
Conversely, I've since realized that the Vietnamese that I think I understand is probably at the toddler level. For a very long time, I just assumed the Vietnamese language lacked features such as pronouns and articles. But then I realized that when my parents tell me to go wash the dishes, my brain just fixates on "wash dishes" and ignores all the other connective words. So I know a lot of verbs and nouns but very few words that are part of everyday conversational speech in Vietnamese. I imagine that's what Alexa feels like :)
Your comment made me curious how well the Google Translate app can deal with foreign languages. I was stunned to see that it could understand my attempts at Vietnamese. So other than a proportionately smaller dataset to learn from (Vietnamese usage vs English usage of Google or Amazon), seems like Alexa and Google Home could competently deal with foreign languages.