Internationalization and localization make it possible to use CiviCRM in languages and cultural environments other than American English. This includes the menus, form labels, help texts, but also date formats, currency/number formats, address formats and supporting various alphabets (ex: alphabetical order of search results).
Internationalization (i18n) is a set of mechanisms to make it possible to translate (localize) CiviCRM to other languages without changing code.
Localization (l10n) refers to using CiviCRM in another language than US English. This includes translating the text of the user interface, but also date, currency and other settings.
Multiple language support (multilingual) means that the CiviCRM instance supports multiple languages, such as bilingual or trilingual sites. For example, on a Spanish/English bilingual site, a contribution form will have a title, introduction text and other configurations that must be in both languages. This means that not only the CiviCRM system messages must be translatable, but also some of the configurations (title, introduction text, etc).
For simplicity, we often refer to all of the above simplify as "translation".
Recent and future changes related to translation are documented in the roadmap.
User/admin guide translations
The user and administration guide has been translated to the following languages:
If you would like to translate or contribute to an existing translation, please join the translation channel on the mattermost chat.
The administrator guide explains how to set up CiviCRM to run in one of the other supported languages. This includes the installation of language files (civicrm-l10n.tar.gz), how to update those files between releases, the various language settings and performance tips.
Since there are a lot of strings in CiviCRM, the translations themselves are managed on Transifex.com. In many ways, it is the "Github/Gitlab" of translation. You can use the Transifex web interface to join a translation team (or create a new one) and translate the strings or improve existing translations from there. Extensions can be translated as well.
Not currently in use: Internationalisation mailing-list: Translation leads should subscribe here, as well as anyone interested in working on the internationalisation code. For support, please use the forums.