Show only distinct TranslationDebug entries

Currently the TranslationDebug log output can get really crowded since every entry in the TranslationDebug is used. This change adds a filter to display untranslated entries only once.
Hi @thorsteneckel, thanks a lot for your suggestion! I don't want to be too picky, but don't you think that using a %Seen hash would be much better from a performance POV? Could you update this?