I have to tackle the lack of this criteria now. My plan is to write a php script going through all users and adding them to specific secondary usergroups depending on selected language, then set up some promotions to handle same thing from now on. Then I can use those usergroups as criteria for the mailing list. The promotion system afaik works only on active users, thus the need for custom script.
It might be simple for someone with php knowledge and time on his hands, but still I feel all this is complicated and a waste of effort that could be avoided by adding this feature.