That is completely unnecessary assumption. People often achieve native level accent on their second languages. And Worf certainly was raised in Russia, as was his brother. Both have perfect American accents. Uhura's mother tongue is Swahili. Geordi is from Africa too. They have perfect American accents. Star Trek is full of character's whose native language is not English, yet they have perfect (usually American) accents.
This is interview of actress Eva Green. Sounds British to me. She is French (there might be some hints of that in few words, but it's pretty much not a stereotypical French accent.)
This is Alexander Skårsgard. A Swede. Sound pretty American to me. And before you say that only actors learn language that well, not true, many normal people do as well. Now, I personally have typical Scandinavian accent (despite trying to get rid of it), but my SO (also a Finn) has a perfect British accent, easily good enough to fool the native speakers.
I only mentioned the American accent because people only find it odd if a second language speaker happens to have British accent but not odd if they have an American accent.