Translations in The Mana World are handled by the [gettext] system. This page presents an overwiew in the context of the game. Please refer to the [manual] for more details.
Gettext for users
Translations are supposed to work out-of-the-box. If they are not, complain to the maintainer of your binary package. This section is dedicated to users compiling tmw themselves.
When configuring, the script should tell you that Native Language Support (NLS) is enabled. It just requires the presence of gettext on your computer. From there, just compile and install like you usually do, and it should work just fine. You just need to have properly set up your system with respect to locales. If console applications, e.g. man, are translated, then The Mana World will also be.
Note that the install part is mandatory for gettext to work. You cannot run the program from your compilation directory and expect it to be translated. Here is a work around:
./configure --localedir=$PWD/locale # plus your special options make # compile as usual cd po ; make install # whenever your .po file is modified