4Spell stores many of its settings in files that can be updated, extended and changed to suit your personal needs or proferences. In the "Language" tab sheet you can select a language dictionary and edit the language dependent files:

After you have clicked on one of the buttons the list is displayed and you can edit the file save it.

Just like in 4TeX, the user-interface language of 4Spell can be changed on-the-fly. From the "Screen language" field you can select a language.