In the German localization, K9 (version 9.0b3) displays [error and other] messages that use the wrong language register, namely informal.
Formal language would be highly advisable. In German, formal and informal register uses different words (which is different from English). Directly addressing the user in English is in both cases "you", while in German formal is "Sie" and informal is "Du".
Addressing strangers in informal language without (prior) permission can be considered offensive, even in a legal perspective. Anyway, it is clearly rude to use informal language without consent...
