I haven't even installed the plugin yet, but I thought of something already..
A 'Special Notice' that is optional to enable with all other languages on the forum...except for the primary language of the forum.
Maybe something like "Our forums were originally designed for an English audience. So if translations are not a 100% accurate in your language , we apologize for this inconvenience. "
This way people will know beforehand if things don't sound a 100% accurate when viewing the forum...
Also, they wont think that everyone posting on that forum is illiterate. lol