It depends a lot on how close the pronunciation of the languages in general are - like every native english speaker will be instantly recognizable even if they speak perfect finnish, no matter how well they learn it, but take an estonian and they might be able to learn so well you don’t spot them. If the phonemes, intonation etc. are completely different, it’s almost impossible to get it right unless you learn the language as a child or speak a cousin language as a native