If you want to translate Mp3tag, just install the latest Development Build in your prefered language and copy the file English.lng from from the lang directory in Mp3tag's program directory to <YourLanguage.lng>.
Please note, that the order of the keys in this file must be preserved (_C_OK and so on), that placeholders like %artist%, %album% should not be translated. Other placeholders like %s and %d are filled with data at runtime and have to be preserved too. Please also note that you have to preserve the order of multiple placeholders (e.g. "%d and %s" needs to be in the same order in the translated string).
The language file is encoded with UTF-8 with BOM (Byte Order Marker).
Please don't hesitate to ask, if there are any questions!
Thank you very much!