docs: Add a Japanese translation style guide.

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Co-authored-by: Greg Price <gnprice@gmail.com>

docs/translating/index.md

@@ -12,6 +12,7 @@ finnish
 french
 german
 hindi
+japanese
 polish
 russian
 spanish

docs/translating/japanese.md

@@ -0,0 +1,15 @@
+# Japanese translation style guide
+
+## Rules
+
+- Use full-width parentheses `（）` for half-width parentheses `()` in original text.
+
+## Terms
+
+- alert word - **キーワード**/**キーワード通知**
+
+  "Alert" can be literally translated as "アラート", however, it gives a negative impression in Japanese. To describe the function clearly, use "キーワード" or more fully "キーワード通知".
+
+- narrow - **～だけ表示**
+- subscribe - **フォロー**
+- subscriber - **フォロワー**

docs/translating/translating.md

@@ -189,6 +189,7 @@ translators can understand and preserve those decisions:
 - [French](french.md)
 - [German](german.md)
 - [Hindi](hindi.md)
+- [Japanese](japanese.md)
 - [Polish](polish.md)
 - [Russian](russian.md)
 - [Spanish](spanish.md)