There is no need to rebuild the application for a particular language. When the application is built, all the messages files in all the available languages are included. The default locale can be changed by setting the application property unitime.locale through the Administration > Defaults > Configuration page.
For a particular http session, the locale can be also changed using the locale parameter, e.g., see our online demo in Czech: http://demo.unitime.org/UniTime?locale=cs. Please note that only some portions of the application were made localizable (and have Czech translations) so far.
Most of the UniTime application was written using Apache Struts, however new pages and components (and in future the whole user interface) are written using Google Web Toolkit (GWT). For localization of GWT parts we use its i18n capabilities.
Here is an example of a change localizing a particular page. The default messages are in English, the language-dependent messages are in a property file with the same name (plus identification of the language using two character iso language code), e.g., see Czech translation of CourseMessages.
Struts localization files are available in org.unitime.localization.messages package, GWT localization files in org.unitime.timetable.gwt.resources package.
Also, please take a look at classes ExportMessages and ImportMessages that can be used to convert existing messages to a CSV file and back -- the file can be easily edited in Excel (or Google Docs) -- it has three columns: message name, English (default) variant and localized variant. One of the first steps could be translating these course messages to your language and trying to plug them in.
Note that the language has to be added in the UniTime.gwt.xml (as a supported language, extending locale property) for the GWT compiler to create the appropriate language mutation.