Language setting for the app provider

[wkloucek]

Is your feature request related to a problem? Please describe.

Opening a file with the app provider does not use the language of oC Web because there is currently no way to pass the language as a parameter.

Describe the solution you'd like

oC Web passes a parameter to the /app/open endpoint with the language, the use has configured for oC Web.
The language option will then be added eg. to the WOPI open parameters (UI_LLCC / lang) or the respective parameter for other apps (but transparent to oC Web).

If the language is not provided, the current default applies. Currently some apps try to detect the primary language of the browser, other apps have hardcoded defaults.

Describe alternatives you've considered

oC Web injects the language into the apps directly. Downside is that oC Web needs to do how to do it for every app.

Additional context

[kulmann]

If you can decide the naming of the additional param for the open request we can already pass it from web.

[wkloucek]

If you can decide the naming of the additional param for the open request we can already pass it from web.

I created PRs in REVA and oCIS that will add the lang parameter documented here: https://github.com/owncloud/ocis/blob/1646b0b5af98747d16d0a54c200f437254f435d9/docs/services/app-registry/apps.md#open-a-file-with-the-app-provider (coming from #4399)