Does "a language they can understand" imply a language they know well?
If not, maybe it's better to add the word "well" or something.

I'm not sure if it's important enough to be worth asking to change the translations, though.

I think it just means that they can understand (often with the help of dictionaries etc) the sentence they are about to translate into their native language. Because they are translating into their native language the translation will be natural-sounding.

I just thought it wouldn't be a good idea if someone translates a sentences of a language they don't know very well. The sentence will sound natural, but it's more likely to be wrong as a translation. That's why I suggested adding "well".

But adding it might make this sentence look too elitist and discouraging. That's of course definitely not what we intend.

There is an instruction warning the person translating not to do so if they don't understand the sentence they are about to translate. For example I know German (perhaps "well" or perhaps "less well') but I only pick out the German sentences I can understand to translate.
We might be able to make that another sentence - something like "When translating from a foreign language into your native language, please make sure you only translate the sentence if you are sure you know what it means" or something like that.
It shouldn't really be necessary, though, as the warning is already there.
Having said that, I'd be happy to change my sentence above to 'well' if others feel it is necessary.

CK. I have changed the sentence. What happens about the Esperanto and Russian translations which are based on the old sentence, before it was amended?