While checking the maillist server answers, I found that those messages do not have any Content-Type headers... Which seems to be rather wrong. For example, in Russian we use a number of character sets among which koi8-r and cp1251 are the most common. That means that most likely the translation for Russian will be in either of them. Therefor, the lack of Content-Type header leads to the problems on the receiving side, where the default character set is different from the one used for translation.
Here is a small patch, which, hopefully, fixes the problem.