Where do you store your text now? If, as I would hope, you have your text in templates (whether using Text Template, HTML::Template, or another similar solution), those templates are going to be stored either on disk or in a database. If it's in a database, simply add a new column for language (I suggest using ISO-standard language identifiers, either 2-letter (en), or 4-letter-with-middle-underscore (en_US), or 4-letter-with-middle-underscore-dot-encoding-name (en_US.iso88591)). If it's on disk, then use the language identifier as a directory name instead.

Also, don't forget that if a particular template is not found in the location you want, you need to be able to fall back to a reasonable default. e.g., if the desired files aren't found, and your default language is English, then fall back to English. If it's not there, then you have real problems ;-)

I've always noticed that it's easier to design this in from the ground up than it is to add in after the fact.