Improve findability of the holiday calendar that is relevant for you

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

In the holiday calendar modal, currently you need to scroll through the list to find your relevant country (if you don’t live somewhere starting with "A").

Describe the solution you'd like

It would be nice to have the relevant calendars sorted up top. We do have the info of locale and language, so we can:

• Sort the first one based on locale
• and the second one based on your language (if different)

According to @ChristophWurst this would need a manual mapping of locale/language codes to the list of Thunderbird holiday calendars. Seems like a 1-time manual task which would improve UX a lot though. :)

Describe alternatives you've considered

An alternative proposed by @nimishavijay was adding a search field. While that will catch more cases like adding other holiday calendars, I think it’s first important to focus on making the main case on adding a calendar for yourself as simple as opening the modal and clicking.

Additional context

https://github.com/nextcloud/calendar/pull/5167

Start by reviewing the holiday calendar modal and the related context in nextcloud/calendar#5167. Identify how locale and language are exposed and how the Thunderbird calendar list is assembled. Done means the locale-relevant calendar appears first, followed by a language-relevant calendar when different, without removing the existing list.