Translation:The message is written in a language that they do not understand.
Because ".., de ikke kan forstå" is a dependent clause.
Yes, you can.
I thought that "er skrevet" should be translated as "has been written". Surely "is written" would be "bliver skrevet".
Is it wrong to write "et sprog, der de ikke kan forstå"? Or something like that with the conjunction included?
"Der" doesn't really work here, but "som" is accepted in the place you wrote "der". "Der" only works as the subject of the subordinate clause