Deprecation: #99531 - Backwards-compatible language key mapping

See forge#99531

Description

Before TYPO3 v4.0, TYPO3 had inconsistencies in its language keys, such as "ja" (= Japan) instead of "jp" (= Japanese), which was still applicable. However, the old language keys have not been in use for TYPO3's built-in translation servers.

In recent years, it was not even possible to use these keys for custom label files anymore.

However, the mapping was still used for detecting the language from the user agent, primarily for the backend login screen when no language was detected.

For this reason, the method Locales->getIsoMapping() has been deprecated.

Impact

Calling the method above will trigger a PHP deprecation warning.

Affected installations

TYPO3 installations which have been maintained for more than 15 years, still using this method, or still using this legacy language keys.

Migration

Migrate to the official language keys / locales by renaming the language files. It is still highly unlikely that these "old" language keys worked in the past major versions of TYPO3.