There are several methods to create a translation, most of which are outlined in the WordPress Codex. In most cases you can contribute to the project on translate.wordpress.org, however for creating custom translations you can use wpForo POT file and PoEdit.
wpForo uses PO and MO files to manage translations. wpForo includes a language file (wpforo.pot file) which contains all of the English text. You can find this language file inside the plugin folder in wpforo/wpf-languages/ directory. The wpforo.pot file can be imported into PoEdit to translate.
To get started open PoEdit and go to File > New catalog from POT file. Once you have done this, choose wpforo.pot and PoEdit will show the catalog properties window.
Enter your name and details, so other translators know who you are, and click ‘OK’. You will be prompted to save your .po file. Name it based on what you are translating to, i.e., a IT translation is saved as wpforo-it_IT.po . Now the strings are listed. After translating strings, you can save. The wpforo-it_IT.mo file will be generated automatically.
These two files should be located in /wp-content/languages/plugins/ directory or better use Loco Translate plugin to relocate the translation files in a safe directory, e.g. in /wp-content/languages/loco/plugins/. Please make sure the po/mo files have wpforo- prefix, without this prefix they’ll not be processed.
Forum backend and front-end language will be changed once you changed WordPress language to the same language in Dashboard > Settings > General admin page.