fix: change locale code for Japanese to ja

[kei95]

Please verify the following:

• [x] yarn test jest tests pass with new tests, if relevant
• [x] yarn lint eslint checks pass with new code, if relevant
• [x] yarn format:check prettier checks pass with new code, if relevant
• [x] README.md (or relevant documentation) has been updated with your changes

Describe your PR

I'm so sorry I just realized the locale code for Japanese was ja while the shortened name of the language itself is jp🙏 This PR fixes the locale applied in the i18n.ts to fix the bug that jp language isn't recognized by i18n. I sware this is the final PR for the series of translations