If I understood correctly that the mod itself has russian subtitles and the mod author has not changed them, you can't change that with settings, you would have to modify the mod itself. The language setting only affects the vanilla game and DLCs, not mods. Because if someone has made a mod using the GECK with a different language, everything in that mod will be in his language unless he changes it manually.
The mod author has probably only changed the voice to english but not the subtitles. The only way to get english subtitles in his mod is to manually edit his mod and replace the russian text with english text.
You might even sometimes have it happen with mods that vanilla location names turn into a different language because of a mod. It's because of the same thing.
I hope this helps and is at least somewhat comprehensible, I might not be the best at explaining this in a clear way.
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