If you want your reddit interface to be in Japanese, you can now go to https://www.reddit.com/settings/preferences and change the 'Display Language' to 日本語. You might have seen it applied already if you are in Japan or have your device language set to Japanese.
However, it seems to be an AI translation. Overall it's fine, but there's some unnaturalness (eg the prompt to write a comment being 会話に参加してください). You may still learn some new words, at least.
by Hazzat