[BUGFIX] Name "default language" consistently

When working with the "pages.l18n_cfg" bitset flag, there is an
option "default translation", which commonly refers to language
with sys_language_uid=0. In most other cases, also in various
documentation pieces, the naming is "default language" instead
of "default translation" which is more correct in this case.

All places are now properly named "default language"
and no "default translation" is there anymore, making the
functionality more consistent.

Resolves: #89924
Releases: master, 9.5
Change-Id: Ibe9ad9371615119eff7535cb45d0f25b858d04da
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62602


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Alexander Schnitzler <review.typo3.org@alexanderschnitzler.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Alexander Schnitzler <review.typo3.org@alexanderschnitzler.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>