なんでドキュメント見ないかなあ。
以下のとおり、utf-translate-cjk-mode では katakana-jisx0201 は対象外。

utf-translate-cjk-mode is an interactive compiled Lisp function in `utf-8.el'.
(utf-translate-cjk-mode &optional arg)

Toggle whether UTF based coding systems de/encode CJK characters.
If arg is an integer, enable if arg is positive and disable if
zero or negative. This is a minor mode.
Enabling this allows the coding systems mule-utf-8,
mule-utf-16le and mule-utf-16be to encode characters in the charsets
`korean-ksc5601', `chinese-gb2312', `chinese-big5-1',
`chinese-big5-2', `japanese-jisx0208' and `japanese-jisx0212', and to
decode the corresponding unicodes into such characters.