First please forgive me for my bad english.
I just wonder how could the problem of à common linguage could be an issue for the purpose of using mentem's spell.
For example, does the magus need to understand the linguage of his target to successfully cast "Posing the silent question"?
I would say, yes and no. That is, some part of "thought" is non-verbal words, at least for me, and I assume others, and a Mentem spell would not translate those for you (In canon, Magic cannot create information. Thus the lack of a translating spell, or a spell to recover words on a faded scroll.). Other parts of thought are memories and images, and those would come through just fine. So, not an "end run" around the rule against translation magic, but still better then nothing......