I see one big problem with translating system expressions. If you're asking for help on the forum (which is english only) providing screenshots, or expression snippets, nobody could help out unless they knew the language.
And besides that Translating basic expressions are not going to help non-english speakers in their future endeavors of developing games as I don't know of any language that translates their syntax commands to local language. So not really a good idea...
Keeping it english would probably be a good things even for the young ones as they learn a few english words along the way.
+1
However Ashley told me, that the user can switch between english and translated versions of expressions.
I personally would remove all expressions from the translation at all.
But as long as they are in, I'll translate them.
Regards
Andy