I am looking for a (simple) text editor that can handle text in different encodings in the same document.
I need to develop some sites with mixed Japanese and Englis
EmEditor is written by a Japanese company for exactly this purpose. It is a fine text editor with good performance/simplicity but pretty much all the features expected of a capable editor; I use it as my default when on the Windows platform, as well as for editing Japanese web page templates. It deserves to be better-known IMO; it is at least as good as, say, TextPad, but with full Unicode support.
Unfortunately it is not free, however you can find a free version of the old EmEditor 6 at sites such as download.com.
For japanese, Sakura Editor is exceptional. It can display UTF-8, EUC-JP, SJIS and so on.
Just to add another one: I just checked that Programmer's Notepad 2 has some UTF-8 setting too.
(vim and emacs do just fine as well)
You can use just Notepad.exe with the "Arial Unicode MS" font (if all of your text is left-to-right, given the English windows version). Just Save as, select UTF-8.
In general, use your favourite editor with a font like "Arial Unicode MS". I mention this one because is the font with the greatest Unicode coverage I have seen,
I like jEdit for it's ability to ident wrapped lines. Really nice when editing XML files. A word of warning though: It's Java, so it's not light fast, like you would expect a text editor to be.
Text codecs are fully supported. It distinguishes between text files with and without the header identifying the file format (byte order mark), calling them UTF-8 and UTF-8Y. This is something that I'm missing in other text editors.
Try SciTE http://gisdeveloper.tripod.com/scite.html. It's just great ;)