Feature: #61542 - Add two-letter ISO 639-1 keys to sys_language

See Issue #61542

Description

The handling of the languages is done by the sys_language database table, which is usually referenced via the common sys_language_uid. The commonly referenced ISO 639-1 two-letter-code is only in use when static_info_tables is installed, which brings all ISO 639-1 letter-codes in a separate table. The CMS Core uses a hard-coded dependency on the extension to retrieve the ISO codes where needed, however, already ships an empty and invisible field "static_lang_isocode" which is already supplied by the CMS Core.

As a first step to identify languages by their proper ISO 639-1 two-letter code a new DB field for sys_language called "language_isocode" is introduced, which is used in all places of the TYPO3 CMS Core.

Additionally the new TypoScript option config.sys_language_isocode can be used to set the existing $TSFE->sys_language_isocode variable via TypoScript. Previously this was done via static_info_tables.

The ISO code is also used for the language attribute of the HTML tag. Therefore the setting config.htmlTag_langKey is not needed anymore if it is the same as the ISO code.

Impact

Frontend: The value $TSFE->sys_language_isocode is now filled at any time. It can be set via TypoScript, or is automatically set if the config.sys_language_uid parameter is set > 0 from the language_isocode DB field.

# danish by default
config.sys_language_uid = 0
config.sys_language_isocode_default = da

[globalVar = GP:L = 1]
        # isocode is filled by the respective DB value from sys_language (uid 1)
        config.sys_language_uid = 1

        # you can override this of course
        config.sys_language_isocode = fr
[GLOBAL]

The new field can be used in any TypoScript variable like

page.10 = TEXT
page.10.data = TSFE:sys_language_isocode
page.10.wrap = <div class="main" data-language="|">

Backend:

All ISO code usages based on sys_language in the Backend (FormEngine, Translation Tools) is now done via the new field but still uses static_info_tables as fallback if already in use.

Each sys_language record is now editable with the new DB field to select the respective ISO 639-1 code.

Migration

The upgrade wizard makes sure that the new DB field is filled properly so no deprecation warnings are written if static_info_tables in conjunction with the DB field "static_lang_isocode" was used before. If this field is used in a 3rd party extension, the extension developers and site integrators need to make sure it is switched to the new DB field sys_language.language_isocode.

If a site uses multiple languages without static_info_tables each sys_language record should be modified to select the proper ISO 639-1 code for the languages.