(take this with a pinch of salt) As far as I remember, the problem lies in the way
libiconv works. Multi-byte encodings need a state machine to decode them, and
libiconv prefers to receive entire characters, so you can't just give it half a character in one function call and the other half in the next.
I can think of another two solutions, one is a good out-of-band method, the other is an in-band hack.
Change Terminal Emulator encoding (out-of-band): one is to change the character encoding in your terminal emulator, so its native encoding is Shift JIS. I just checked
konsole, and is supports this. From the menu, View→Character encoding→Japenese→sjis. You can then just
tail -f the file, and
konsole will take care of decoding the multibyte characters and matching them up to font glyphs.
Transcode terminal encoding on the fly (in-band; best): courtesy of Gilles, who reminded me of
luit after a very long time. Use
luit, which should have come with your XOrg distribution (on Debian, it's package
x11-utils). Use it like this:
$ luit -encoding SJIS -- tail -f x
This will make the terminal transcode SJIS to/from your terminal encoding, and run
tail -f x. The downside of
luit is that it doesn't support the wealth of encodings supported by
libiconv. The upside is it's available almost everywhere.
Transcode terminal encoding on the fly (in-band; hack):
ttyconv is a hack I wrote many years ago (initially in C, later redone in Python) which uses
libiconv to transcode terminal I/O. It spawns a new pseudoterminal and (a) transcodes the characters you type from your local encoding into the remote encoding, and (b) transcodes the characters you receive from the remote encoding to your local encoding. I used it to talk to servers that used encodings not supported by the standard Linux terminals. Please note that all of the remote encodings I tested it with were single-byte encodings, so I can't guarantee it'd work for Shift JIS. I don't often find call to use it these days, with most systems switching to Unicode.
This is how you would use it:
$ ttyconv -rsjis -- tail -f x
The downside of
ttyconv is that I wrote it, no-one uses it but me, it's probably full of bugs. I excel at this. The upside is that it uses
libiconv, so if your encoding is unusual, it's your best bet. At last count,
ttyconv --list supports 100 encodings.