Any benefit to writing a message as a mix of 2 languages before encryption? For example, imagine a message that is a mix of Japanese and English words; Japanese in unicode, English in ASCII. Japanese doesn't use spaces between words.

The sentence structures, anything abstract, and verbs would have to be in Japanese. Anything directly translatable such as colors, transportation machines, proper nouns, etc. would be in English. 

Would writing the original message as a mix of 2 languages (especially a Western and non-Western language) contribute to making it more difficult to decrypt?
Historically, there did exist a benefit to using a language that the adversary was not familiar with.  The name for this is code talkers, and the most famous ones (at least in the USA) are the Navajo code talkers of World War II.  The idea was to defeat attacks that relied on statistics about the language used in the plaintext.

In modern cryptography, however, there is no benefit, because modern ciphers are designed to obscure any patterns in the plaintext—even patterns that we haven't thought of.  This is because modern ciphers are expected to be not just "hard to crack," but indistinguishable from random data.

There's a second reason why using multiple languages would not have any benefit, which is that you're using a computer, so no matter what language you use you're still encoding text into a character encoding like UTF-8—and this already gives you statistical patterns that an attacker could look for.

Third reason: English and Japanese are both widely spoken languages with lots of example text available.  There's a reason why code talkers tended to use aboriginal languages.

Fourth reason: Kerckhoffs' principle.  Your cryptographic system should not rely on the secrecy of simple tricks like what you propose.