I have multisite WordPress installation with default language for all new sites set to English. For one of my sites I wanted to add support for different (Polish) language. I copied all Polish *.mo files I found in Polish installation of WordPress.
After switching to that language that one site only, I noticed that not only frontend, but also dashboard has been translated into Polish. Because that was unwanted effect, I deleted all *.mo files from languages directory, except for pl_PL.mo. Especially I removed *.mo files for admin and network admin.
Even though I did it, I can see right now, that my Dashboard for this site is partially translated into Polish. Only certain strings (dashboard sections, frames, buttons) are translated, other remains in English.
What is going on? Form where this partial translation comes out? I only left pl_PL.mo file, which (as I understand) purely for frontend translation and does not changes anything it dashboard.
This case goes for my test blog (network's site), so I can use ultimate solution -- that is, delete whole site and re-create it again. But even though, I'm pretty curious, what has happened, and what to do, if similar problem appear in the future for one of my production blogs?