If two or more of the languages do not share (or do not use) an 8-bit encoding, you should use UTF-8.
PHP has crap Unicode support, I strongly urge you not to use PHP if you need Unicode.
Assuming that your multilingual site will have several languages which share an 8-bit encoding (like Western European, English, French, Spanish, Italian, German), then it should be easy enough.
Example:
OK: English, French, Spanish, German, Italian
OK: English, Russian
Not OK: English, Russian, French (Because the French character set does not contain Russian characters and vice versa)
Really really not ok: Japanese, Chinese etc
Make a file with constants for messages, and define one copy of this file for each language. When you need to choose the language, choose one of these files, include it, then you can use it throughout.
Of course any items stored in the database need to have multilingual versions available, which may make the schema more complicated. But I'm sure you can handle that.
Mark