The best examples that come to mind are when ordering food. As examples:
• You speak English and Spanish and are ordering a burrito
• You speak Thai and English and are ordering Tom Yum
I imagine it could depend on numerous things:
• You primary language or ethnicity
• What sort of restaurant
• Who you’re dining with
• Who you’re ordering from
• and probably a lot more…
Definitely serious. Loanwords fall all across the spectrum regarding how much they’re integrated from their source language into the “local” language and most folks don’t know or care about word etymologies enough to even consider it. If you’re not native there’s a good chance you’re mispronouncing loanwords even when you’re trying to use their native pronunciation.
Loanwords are a different story. I would argue those are part of both languages at that point. Burrito is an English word and a Spanish word. As I see it, anyway.