I doubt that 'thinking too hard' isn't possible in Nation RP. It sort of depends.

The language barrier can be created or ignored. That's something you chose. Urukhai and I seemingly decided that just talking the same language would be easier (we haven't really discussed with it. We just rolled with it). You can chose to do this (so there would be something called the Common Tongue or something like that) and ignore the language barrier.

If you chose to make a language barrier then it doesn't mean there has to be a real problem. You could say that someone on a border town or a trader that comes in ones every three years (for your famous silver maybe?) or so can speak the language a little. He could have been hired by a diplomat from Valhall. (i also think that the term Diplomat is better than just runner).

Or you could say that the diplomat entered the country somewhat unknowing and just following rumors of a nation there. He could be captured, tortured and/or slaughtered. Or there would be no diplomat and you were never discovered. I think Corin really left this all open for you. But remember that you can't stay invisible. I'm sure that with the supposed freedom of Corin, there also comes the price that Valhall will have send a lot of spies out. Meaning that at one point, you will get discovered.

In the end, i think you should try to come up with something and please, never think you think too hard on something. It can make nation RP very fun.