No, you couldn't be compelled to translate a document in a foreign language. The police have another way to get the information - ask someone else who knows the language to do it. You would be making their job harder, but not impossible. In the case of an encryption key, you might be the only one that can assist the investigation. So you'd be hindering it. At least that's how I'd see it.
The accused will often be the only one who knows how strong the encryption is, or how prevalent the foreign language is, so they're the only one who knows whether it is possible for the police to decrypt/translate without their cooperation. Thus, in reality, the determination of whether the accused can be compelled to provide the decryption information cannot depend on the strength of the encryption/obfuscation.
What if the language has very few speakers left in the world, including you? If only you speak the language, could they compel you to translate it? Or if only you and one other person in the world spoke the language, and neither of you wanted to translate it?
If it isn't a real language, that's a whole other set of circumstances. If it is a foreign language that is actively spoken, and the police have a reasonable way to get the document in question translated, you should have to help them. However, if they don't have a reasonable way and you are the only one that can help, then you could be compelled to do so. It is a question of what's reasonable.
