docs: simplify translation links in README_ja.md

Author: nkmr-jp

README_ja.md

@@ -1,14 +1,6 @@
 <!-- Keep these links. Translations will automatically update with the README. -->
 [English](README.md) |
-日本語 |
-[Deutsch](https://readme-i18n.com/nkmr-jp/prompt-line?lang=de) |
-[English](https://readme-i18n.com/nkmr-jp/prompt-line?lang=en) |
-[Español](https://readme-i18n.com/nkmr-jp/prompt-line?lang=es) |
-[français](https://readme-i18n.com/nkmr-jp/prompt-line?lang=fr) |
-[한국어](https://readme-i18n.com/nkmr-jp/prompt-line?lang=ko) |
-[Português](https://readme-i18n.com/nkmr-jp/prompt-line?lang=pt) |
-[Русский](https://readme-i18n.com/nkmr-jp/prompt-line?lang=ru) |
-[中文](https://readme-i18n.com/nkmr-jp/prompt-line?lang=zh)
+日本語
 
 # 🧑‍💻 Prompt Line
 [![Ask DeepWiki](https://deepwiki.com/badge.svg)](https://deepwiki.com/nkmr-jp/prompt-line)