iMIP invitation sent with organizer's preferred language if no language set for attendee

[ChristophWurst]

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Steps to reproduce

0. Set the instance default_language to 'en'
1. Set your language to something distinctive, like French
2. Configure your calendar in Thunderbird
3. Create a new event, invite an email attendee
4. Save the event

Expected behavior

The invitation is sent out in English

Actual behavior

The invitation is sent out in French

Ref #12020
Ref #11795 (comment) @skjnldsv @oparoz

[ChristophWurst]

Some debugging findings.

The magic happens in \OCA\DAV\CalDAV\Schedule\IMipPlugin::schedule. \OCP\L10N\IFactory::findLanguage gives me fr as "default language" because the user has set their lang is set to fr. The global default_language is never taken into consideration.

[ChristophWurst]

\OCP\L10N\IFactory::findLanguage claims to find the best language. I would argue this method finds the "best language" for the current user. We need something that finds the "best language" for the instance. The logic would be a bit of the opposite. It would first check if there is a default_language set, then it takes that. Then it can check if the current user has a lang set and use that, last it will use en. @nickvergessen what do you think? :)

[nickvergessen]

Should first check force_language (still a thing?), then default_language, then sender and final en.
Lots of emails are doing this manually (activity mail) or should do (Talk email invites), so would make sense to have a dedicated method on the IFactory I think?

[ChristophWurst]

PR at #29015