I envy my daughter. She learns languages easily in the classroom. I could never do that. If I have no immediate need for a language I can't learn it just because I want to. I spoke VN pretty fluently in 68-70 and since the early 1980s have been very close to the local VN emigre community. I go to mass in Vietnamese. That has not kept me from losing most of the language. I got books and later got computer programs. I paid a lady to give me lessons. No good. The emigres were/are all trying to learn English and I don't have to speak VN. In one month back in Viet Nam last summer I got pretty good again. No one around me spoke English at all. When I got back I was talking to my friends like they were in the old country. Now I'm losing it again.
You know, I thought that "Thanh" looked Vietnamese.
I can't learn any language, classroom or otherwise. My ears don't catch sounds very well and my mind doesn't process aural information that quickly. Add to that the fact that spoken languages are taught inductively and there you go.
The one and only language I have ever learned is Biblical Hebrew, which is a textual language taught deductively and learned visually (letters on a page). Plus I had almost supernatural motivation to learn that one. It remains to this day my one and only language success; I haven't even been able to learn Israeli Hebrew.
There is something mysterious and incantational about liturgical languages that makes them more appealing than mere everyday conversation in another language.