I did not understand why the "ikke" word is in second position....
Because ".., de ikke kan forstå" is a dependent clause.
Also we can insert 'som' before it?
Yes, you can.
I thought that "er skrevet" should be translated as "has been written". Surely "is written" would be "bliver skrevet".
"Bliver skrevet" would be "is being written"
I believe here "skrevet" is acting as an adjective rather than a verb
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
tak! Jeg havde glemt forskellen mellem "der" og "som".
I put '...that they cannot understand'. Why is this not correct?
Dansk rhyme time. Jeg kan godt lide, hvordan sætningen ruller ud a tungen.
It looks like cannot understand. If that is wrong, then how would you say "cannot understand"?