Then if you say about pretend, why limit only to the accent? Don't you think that it is pointless?
This isn't a conversation about what accents actors use. It's about whether the characters are portrayed as all-American or as globally diverse. There are plenty of Trek characters who speak with American accents but are overtly not from America -- Uhura (US of Africa), La Forge (Somalia), Yar (Turkana IV, with Russian ancestry), Chakotay (Trebus, with Native/Latin American ancestry), Seven of Nine (Tendara colony, with Scandinavian ancestry), etc. Accents can be one way of coding a character as non-American, but they're not essential for doing so. And thus they aren't the point of this conversation at all.
So if you really care about the character from Russia, then let him / her speak Russian with Russian native accent. Not American Russian Accent. If not, why still need to bother?
Because they don't exist in isolation. They're serving on the same crew, so obviously they're using a shared language. They could certainly use their native languages when speaking with their family members, but then code-switch to the standard/official Starfleet language when they're on duty or conversing with their crewmates, because that's how bilingualism works. And it's not unreasonable that the shared language would be English. It's the most widely spoken second language on Earth, and it's the international language of space travel, science, and engineering, so it's a plausible choice to become the lingua franca of humanity in space. Sure, theoretically it could be supplanted over the next 2 or 3 centuries by Chinese, say, but obviously that's not going to happen in a show made for English-speaking audiences.