Add "language/xx" to "labels" in all i18n OWNERS (#4383)

This commit adds "language/xx" to "labels" key in all
`i18n/xx/OWNERS` files.
By this change, if any files in `i18n/xx/` has changed,
"language/xx" label is added in automatically.

i18n/fr/OWNERS

 approvers:
   - feloy
 
+labels:
+- language/fr

i18n/ja/OWNERS

@@ -4,4 +4,3 @@ approvers:
 
 labels:
 - language/ja
-

i18n/ko/OWNERS

 approvers:
   - seokho-son
 
+labels:
+- language/ko

i18n/zh/OWNERS

+labels:
+- language/zh