Adding a new language or locale
If your language/locale is already present in one of the language list boxes of the font attribution dialog (Format → Character → Font), probably nothing needs to be done, except maybe an entry to the build system needs to be added. If in doubt, just ask on the dev@l10n mailing list.
Adding a language to the language list box makes it available for character attribution and spell-checking. Availability of spell checker, hyphenation and thesaurus of course depends on the installed dictionaries. To be able to fully support it as a locale, including number formats and calendar data, and having it selectable as default document language we would need a locale data file.
Locale mapping
If you plan a new localization of OOo and want to have a new language or locale added, some very basic assignments and mappings have to be done to make it known to the code and the build system.
Adding_a_new_language_or_locale
