Collabora editor UI language should follow the language of the UI of XWiki

[lucaa]

For a user editing an attachment of a french page in a french browser, it's expected that the UI of Collabora is in French, but today it's in English.

The good choice is probably the current UI language of XWiki, which may depend on multiple factors: on a monolingual wiki it is the language of the wiki regardless of the language of the document, on a multilanguage wiki it depends on the current chosen language.
The idea is that the Collabora UI should be coherent in terms of language with the XWiki UI.

[oanalavinia]

Done as part of 17f1d28

Couldn't find an answer on the documentation, but found one on their forum (EDIT: found it in the documentation after the forum post https://sdk.collaboraonline.com/docs/conversion_api.html?highlight=lang )

Note that it's still needed for a language to be installed on the Collabora online server. If your wiki uses fr_CA but this locale is not installed, then it will fallback on the default language (which might be configurable, but it is en by default)

To configure the installed languages, please check their documentation. For example, for docker there is a list of languages (dictionaries) already installed https://sdk.collaboraonline.com/docs/installation/Docker_image.html?highlight=language#create-a-container-from-the-image-and-run-it with the possibility to add more, here is documentation for installation from packages and in here you can see a starting point for where the language options might be on your Collabora server