feat: add Japanese translation (#5816)

# Description
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Please also include relevant motivation and context. List any
dependencies that are required for this change. -->

Closes #5815 

**Type of change**
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PR according to the type of change -->

- New feature (non-breaking change which adds functionality)

**How Has This Been Tested**
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-->

Checked that the UI works with the Japanese language by setting the
language in the local environment. Used
[docker-compose.yaml](https://github.com/argilla-io/argilla/blob/9bc5f101790a8d901884d2ba52a60f7b8f354f38/examples/deployments/docker/docker-compose.yaml)
for backend.

**Checklist**
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account -->

- I added relevant documentation
- I followed the style guidelines of this project
- I did a self-review of my code
- I made corresponding changes to the documentation
- I confirm My changes generate no new warnings
- I have added tests that prove my fix is effective or that my feature
works
- I have added relevant notes to the CHANGELOG.md file (See
https://keepachangelog.com/)

Author: Tomoya-Matsubara

argilla-frontend/CHANGELOG.md

@@ -16,6 +16,10 @@ These are the section headers that we use:
 
 ## [Unreleased]()
 
+### Added
+
+- Added Japanese language ([#5816](https://github.com/argilla-io/argilla/pull/5816)). Contributed by @Tomoya-Matsubara.
+
 ## [2.7.1](https://github.com/argilla-io/argilla/compare/v2.7.0...v2.7.1)
 
 ### Fixed

argilla-frontend/nuxt.config.ts

@@ -123,6 +123,11 @@ const config: NuxtConfig = {
         name: "Español",
         file: "es.js",
       },
+      {
+        code: "ja",
+        name: "日本語",
+        file: "ja.js",
+      },
     ],
     detectBrowserLanguage: false,
     vueI18n: {