That would multiply the size of DCS.
At the moment, not so many missions are multi-language.
So instead of X missions we would have X times Number_of_available_languages missions.
That will change once multi-language missions are implemented across the board.
Also instead of Y flight manuals we would have Y times Number_of_available_languages manuals.
I don't feel it's the way to go, to solve your "problem" which is not that common.
A better way would probably to be able to select which language to use per module, but that also would create problems (at the moment, many modules share the same localization files in the sim, for example input translation). So not gonna happen soon.
Finally, a simpler way: get two copies of DCS! I use DCS release in English and DCS open beta in French. Double the disk usage but works great (and I'm one of not-too-many people who, like you, likes to use different languages depending on this or that).
You can do that, or use 2 copies of the same "branch" (Release, OB or OA).
Regards,
Az'
[edit] wow, basically same answer as cichlidfan; we posted at the same time ;)