These are used in UFO:AI to let people play the game in their own language. The number of languages available can be seen as a measure of the game's success.
compiling po files
This must be done in order to play a bleeding edge version from the svn repository (at least with UFOpedia, intros, mail client, ... working properly).
- get poEdit.
- find the *.po file for your chosen language. eg \trunk\src\po\fr.po for French.
- save. poEdit defaults to compiling the po and saving a mo.
- move and rename the mo to something like \trunk\base\i18n\fr\LC_MESSAGES\ufoai.mo. Again the fr directory here means this example is specific to French.