Well the translation spell effects your ability to understand what they're trying to communicate. You have some basic structure when it comes to messages in communication.
Sender > encoding >---message--->decoding> receiver
From what I'm gathering, this spell would have the receivers ability to decode a message simply align itself perfectly with the encoding message, so you know exactly what they're trying to say, being so effective as to even cross the language barrier. Even if you don't know the language, your ability to interpret what they mean makes your necessity to learn the language a moot point.
Or am I misunderstanding your questions?
Written languages wont be affected, because the spell covers the hearing sense. I thought the wiki said that?
This means Nukian as a sign based language cannot be interpreted by those who do not already know it, functioning like a language we know in real life. As for how far the spell covers, I've imagined it as a chain reaction, flowing along the streams of magic in Felarya, so its strongest where there is lots of magic, and weakest for where there is less. This is why magic poor zones like Mycoria doesn't implement it as effectively.