VPOP3 has two places where internationalisation can be carried out.
In the VPOP3 directory on the server are some *.LNG files. These are language files for the WebMail interface. You can create a new language file by copying the ENGLISH.LNG to another file, eg FRANCAIS.LNG, and then use a text editor (eg Notepad) to edit the text strings in there (the text after the '=' on each line
In the VPOP3 settings, at Settings → VPOP3 Text Strings, you can customise the text strings for some messages generated by VPOP3 in there. Any customisations are stored in the MESSAGESTRINGS.TXT file in the VPOP3 directory, so you can copy that file to another computer to copy the translations to the new PC.
Here are some language files which may be partially completed. Users can edit or submit the entries here to make them publicly available. Other users can copy/paste the text to a suitable <language>.LNG file to use the translations
(Keywords: change text, changing error message, customize, translate, internationalize, internationalise, translation, nls, edit, editing error messages)