If you’re not familiar with any translation methods and you want to quickly translate forum front-end phrases you should use wpForo built-in phrase system. In Dashboard navigate to Forums > Phrases admin page and you’ll find a full list of wpForo front-end phrases.
Here you can edit each phrase one by one and translate to your language. If you can’t find some phrase you can add it using [Add new] button.
IMPORTANT: If you did some change in phrases, please don’t forget to delete phrase cache in Dashboard > Forums > Dashboard admin page.