Try removing the utf8_from_wstring from g_L10n.Tranlsate for a string that is known to contain an UTF8 symbol in the pology file and see that the utf8_from_wstring is necessary for translated strings.
See gloox specs to see that gloox in fact uses UTF8 for strings
Make sure that every utf8 conversion is applied where necessary.
Make sure that every string copy is avoided that can be avoided, or find a good reason to copy.
Test all of the GUI messages with two player accounts and a mod account.