BabelPad is powerful, portable text editor with strong Unicode support, which should be able to properly display files in just about any language.
As you'd expect, there's support for opening files in a lengthy list of encodings: UTF-8/16/32/7, ISO-8859-1 through to ISO-8859-16, assorted Windows CP's (Thai, Traditional/ Simplified Chinese, Korean, Cyrillic and more), and the list goes on: JIS X 0208 (Japanese), KOI8-R (Russian), ARMSCII-8 (Armenian), TSCII (Tamil) and more (see the BabelPad site if you need the full list).
If you're editing some really complex document then you'll appreciate BabelPad's ability to assign different fonts to individual scripts, so you can more easily work with multi-script files.
Elsewhere, the program also provides input methods which allow you to enter a variety of scripts: Tibetan, Manchu, Mongolian, Uyghur and Yi (Nuosu), as well as the ability to manually enter Unicode characters by entering their hexadecimal values. You can switch between left-to-right and right-to-left page layouts in a couple of clicks, and the use of Microsoft's Uniscribe rendering engine ensures everything looks as it should.
And let's not forget that despite the nod to Notepad in the program name, BabelPad is actually a surprisingly powerful text editor, even for plain ordinary English-language files: there's optional auto-indent, powerful find and replace, drag-and-drop support (select some text and you can drag and drop it anywhere else), multi-level undo and redo, and more.