Translation templates are translation files that contain only the source text (original text). These files are used as a template to create target files for each language.
Users familiar with Gettext know translation templates as POT files. For other bilingual formats (like XLIFF) untranslated files with the same extension are used as templates.
Pootle can manage a special language called templates. This is not strictly speaking a language but rather a place to store translation templates for a project.
If the Templates language is present then Pootle will initialise brand new languages with files from the Templates language.
If the Templates language is absent from a project, Pootle will assume all initialisation of files for new languages happens outside of Pootle.
It is helpful to understand in more detail how a new language is created or added to Pootle.
When adding a new language to a project from the Pootle interface, Pootle will first scan the file system and look for translation files for that language. If they exist then these are imported into Pootle. If no files are present and if the Templates language exists then a fresh copy will be generated based on the templates files (in a manner similar to pot2po).
If there is no Templates language it is usual to manage all initialisation of
languages from the Pootle command line. When using
new languages will be initialised if they are present on the filesystem. You
are responsible for initialisation of these new languages from template files
Pootle will not update existing translations if new template files are added to Pootle. Updating of translations is managed outside of Pootle. You can update your translations as follows:
sync_storesto sync all translations to the filesystem. These files will now contain the latest translations from Pootle users.
update_storesto push the updated translations to Pootle.
A detailed example can be found in Updating strings for existing project.