V8: Allow localization of the backoffice using parent cultures to the editor culture

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

If there's an existing issue for this PR then this fixes #5959

Description

This is a new take on the issue outlined in #4519. The PR reverts the changes from #4519 and handles dictionary based localization via region-less languages in a different way.

Setup for testing this PR

First and foremost make sure you have created both a region specific and a region-less language - e.g. add "da" if you still have the default "en-US":

Create a doctype that localizes both its name and a property label via the dictionary (using the "#" notation):

Add two dictionary entries, one for the name of the doctype and one for the name property label:

Testing this PR

Set your user language to match the region-less language you created above - e.g. use "da-DK" as your user language if you created "da".

Verify that the localization works by testing the following:

1. The doctype name in create dialog:
   

2. The property label in the doctype editor:
   

3. The doctype name in the "Info" section:
   

Now change your user language to match the region specific language you created above and redo the verification of localization for this language.

[nul800sebastiaan]

I checked all the usages of this now a couple of times and this is a much better fix in the first place. Good find of that other method, it seems to be exactly made for this and we don't have to break the other one either (or at least.. it's being "unbroken" now!).

Thanks very much! 👍