I use the Private Messages addon.
Translated to Hebrew most of it, but can't make the main "Send a Message" button show my translation.
Another thing is that the button is not located well, and is going out of the page (screenshot).
All wpForo phrases are located in Dashboard > Forums > Phrases admin page. Just find the phrase and translate it to your language.
If you can’t find some phrase you can either add it using [Add new] button or navigate to Dashboard > Forums > Dashboard admin page, click on "Rebuild Phrases" button and try to find the phrase again.
Please don’t forget to delete phrase cache in Dashboard > Forums > Dashboard admin page.