suggested tools for simple i18n

Hi All,

I making some web-based tools and I need to support different languages, 
at least european and japanese.

The task is fairly simple: I only need to replace certain static strings
based on the language chosen. What is the easiest way to do this? 
gettext? mbstring?

Could I simply have a bunch of php-files with the constants in them, one 
file per language?

Thanks in advance

-Thorsten