For an example of program that uses a single language file, you should take a look to one of the Dada Mail Competitor : WAnewsletter.http://phpcodeur.net/wascripts/wanewsletter/
It's a French program but once installed, it's multilanguage.
This PHP web system use a single language file for the whole program. You want a new language ? Just take a tour on their web site to see if it's not already available, else, you can just translate the file and declare it in the program.
You're done !
On the other hand, WAnewsletter seems much less user friendly than Dada Mail. It does not use Ajax, and is not maintained since oct 2006.
But I had to choose WAnewsletter because of the language : I installed it on a spanish website where the ones in charge of sending mails were only spanish speaking persons. They needed the whole interface in spanish.
You should really concentrate your work on the multilanguage option since obviously a lot a people need that kind of program and does not speak the Shakespeare language.