Japanese (platex) works fine with fontenc.

While not required for hyphenation, using T1 as default font encoding
helps with text in Latin script (pre-composed accented characters,
Nordic letters "eth" and "thorn").

lib/languages

@@ -862,8 +862,7 @@ Language japanese
 	Encoding         jis-platex
 	LangCode         ja_JP
 	Requires         japanese
-	InternalEncoding true
-	FontEncoding     None
+	FontEncoding     ASCII
 	DateFormats      "yyyy年M月d日|yyyy/MM/dd|yy/MM/dd"
 	QuoteStyle       cjk
 End