Fix root cause of language problems

[BPerlakiH]

After throughout analysis, why the AppStore version is working as it is, what were the changes in the meantime.
I did found the following:

• it all started with: Replace deprecated function use #588
• I removed the deprecated function in https://github.com/kiwix/apple/pull/629/files
• The former function was returning a single string, whereas the new one might return a comma separated list of strings.
• The mapping from a list of strings to a list of Locales, was done a "level higher" not in the objective-c function but in swift.
• The result is that the objective-c function was in fact converting the language codes from the feed to a 2 letter language code (across all iOS versions), and that is what we were storing in the database, which is accidentally or purposefully but in an undocumented way is working
• When adding the swift function I was checking against the feed values, and from that PR on the DB was using 3 letter codes.
• Unfortunately the unit-tests were not run by me on that PR, the CI is currently not running them
• The tests were updated at a later point in time here: https://github.com/kiwix/apple/pull/652/files#diff-edd62679239139566e8891207312793f6afaf35c590661caebec8c72f81bbfc3

Proposition

The simplest, and most straightforward solution at this point is, use the existing working solution and store the language codes as 2 letter codes, apply the same for user settings. This solves the issue with iOS 15 and below, without having to manually map codes, as suggested in: #667

This purposeful workaround has to be clearly documented in code.

To avoid similar problems in the future, we need to make sure unit tests are run during CI builds (issue created)

[BPerlakiH]

@rgaudin I put the ISO-3 migration into a PR on top of this, see: #672
I discovered even more things, and implemented in a more consistent way.

[rgaudin]

So maybe close this one?