wpForo Forum language is based on WordPress language set in Dashboard > Settings > General admin page. 

If you changed WP language wpForo already loaded on that language. You can see buttons and other parts (user interface) are already in your website language. If some phrase is not translated you should edit your website current language translation file (PO/MO) in /wpforo/wpf-languages/ directory and complete the translation:

• Using po/mo translation files: https://wpforo.com/docs/translation/translate-using-pot-file/
• Using wpForo Phrase system: https://wpforo.com/docs/translation/quick-translation-phrases/
• Using wpForo Phrase system XML file: https://wpforo.com/community/faq/how-to-translate/
• General information on how to translate wpForo: https://wpforo.com/docs/root/translation/how-to-translate/

For further support please use the wpForo community, here we mostly support issues, related to wpForo add-ons. 

Best regards
Anders H
Sweden