For a language to be supported completely in Debian Installer and later in Debian GNU/Linux, one locale must exist for this language.
This document is not aimed at describing locales and which information they should contain. In short, a locale describes specific properties of a language/country combination. For instance, the "de_AT" locale describes specifics of the German language in Austria.
Information included in a locale:
names of week days (and abbreviations);
names of months (and abbreviations);
official symbol of the currency;
writing style for numbers;
writing style for dates;
writing style for telephone numbers;
usual paper size;
collation information for the given language (one of the most complicated parts of locales);
.../...
The writer of a locale must use the locale files format. The contents of this file must be in UTF-8. Translators shouldn't be afraid by this: this part may be handled by an i18n specialist in Debian, most often one of the Debian Installer i18n coordinators. The new translator(s) will only need to provide him/her the above information when (s)he asks for it.
If the language coordinator is aware of it, (s)he should provide the name of the most used locale(s) for this language. The Debian Installer i18n coordinators will check whether such locale already exists in Debian.
If the new language has no locale, neither in Debian nor from other free sources, writing the new locale will need a lot of work and this step may become very long, especially if it has to be done by one of the Debian Installer i18n coordinators.
For this step to be considered complete, a new locale must either
exist or be requested for addition (through a regular bug report) in
the locales Debian package.
The Debian Installer i18n coordinators will record this information in the supported
languages list by changing the nlp_step field
value to locale.