Add en_DE about holidays

[j-f1]

Seems like it would be best to have holidays by region rather than language.

[viktorasl]

When fixing GitHawkApp/GitHawk#2560 I've read localization docs so the thing is that Locale.current.identifier will be one of Locale.availableLocaleIdentifiers and it will consist of: language code, often includes a region code, and occasionally a script designator.
Which means that if I have set a Portuguese language as a primary on my device and set Lithuania as my region Locale.current.identifier will be equal to en_LT.
Currently in GitHawk app case all identifiers will start w/ en_
Having that in mind there will be lots and lots of jsons which will be the same.

To come up w/ much more simple solution we should decide which is more important: language or region for inbox zero message:

• if language (i.e we'd like to greet people the language they prefer): I suggest taking first value from Locale.preferredLanguages which is a list of device languages;
• if region (we'd like to greet people with holiday in region where they live/work): I suggest taking into account Locale.current.regionCode which is one of Locale.isoRegionCodes (255 values at the moment).

To sumarize Locale.current.identifier does not give enough context if app language is not important.

I personally have my language set to English & region to Lithuania.