Add Japanese translation to multilingual.mdx
Author: shogohida
Pull request: yes

Signed-off-by: Shogo Hida <shogo.hida@gmail.com>

# What does this PR do?
Adds Japanese translation to multilingual.mdx

Fixes #18413 

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## Who can review?

@omarespejel @sgugger 

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

I used the formal form to translate because Japanese is normally written in a formal way in docs. 

I wrote this memo because one of the requirements was to use an informal tone. https://github.com/huggingface/transformers/issues/18413#issue-1325310941

@ArthurZucker can you have a look here? Thanks!