You translate the user interface, radio chat messages, mission editor txt's. Or to put it that way "anything you read" in armed assault apart from briefings. Since it is all defined in the stringtable.
Here a small sample of the file to get a idea:
Head of Stringtable from OFP:
COMMENT,Date and Time,Date et heure,Data e ora,Fecha y hora,Datum und Uhrzeit,Datum a ?as
As you can see the 1st line defines the languages, followed by your STR_ for weekdays. Followed by the days in all the languages you defined in the 1st line under LANGUAGE
Here is a sample of the Games user interface STR_:
COMMENT,InGameUI,InGameUI,InGameUI,Interfaz del juego,InGameUI,
So quite easy to modify if you have a small idea of what you're doing. It can be done with notepad or excel. People keep saying .csv is a exclusive excel file. That's bull****. Fastest edit and best overview is wordpad or wordpad++