Terminal shows non-ascii? characters in UTF-16 hex codes

Question

todoroki@todoroki-VJZ13B ~>printf "ä\n"
                           echo "ä"
                           ä
ä
ä
\udcc3\udca4: \u30b3\u30de\u30f3\u30c9\u304c\u898b\u3064\u304b\u308a\u307e\u305b\u3093

according to a UTF-16 decode tool, \u30b3\u30de\u30f3\u30c9\u304c\u898b\u3064\u304b\u308a\u307e\u305b\u3093 is コマンドが見つかりません (= "command not found"), which is the correct Japanese output I expect.

From the printf and echo result, UTF-8 seems working correctly.

This happens in all shell outputs, such as ls (Japanese characters in filenames shows up in UTF-16 hex format)
EDIT: ls output was not utf-16, but something called "Octal Escape Sequence" (where 月 becomes \346\234\210)

ls in a directory which contains 3 folders named C, あいう, and 月 outputs:

todoroki@todoroki-VJZ13B ~/test> ls -l
total 12
drwxr-xr-x 3 todoroki todoroki 4096 10月  4 15:02  C/
drwxr-xr-x 2 todoroki todoroki 4096 10月 11 09:04 ''$'\343\201\202\343\201\204\343\201\206'/
drwxr-xr-x 2 todoroki todoroki 4096 10月 11 09:05 ''$'\346\234\210'/

(and this is weird because 月 of the file creation dates are shown correctly, while the directory name one isn't)

less vi nano behaves more strange; a file (a.txt, created with gedit) like below

あ
い
う
ä

will show as

in less (it complains "a.txt" may be a binary file. See it anyway?):

<E3><81><82>
<E3><81><84>
<E3><81><86>
<C3><A4>

in vi:

�~A~B
�~A~D
�~A~F
ä

and in nano:

 ^a^b
 ^a^d
 ^a^f

I don't remember what I had done, but it was correctly showing Japanese letters at least two days ago (and for more than 6 months).

What could be the problem and the way to recover from this?

Answer 1

I had accidentally updated my fish config file to read ~/.profile,
which included a line saying locale=C.

I changed this to locale=C_UTF8 and everything recovered.