TYPO3 Core

Hmmm looking into neos/fluid doesn't help us here.

https://git.typo3.org/Packages/TYPO3.Fluid.git/blob/HEAD:/Tests/Unit/ViewHelpers/Format/DateViewHelperTest.php

I would use a new function name like "setOwnLocale()" as (beside PHPUnits setLocale) we don't restore the locale afterwards and using PHPUnits setLocale will throw an exception instead of skipping if locale not found on system.

But I've 2 other small nitpicks while reading:

1.) False locale de_DE.UTF-8 inside Provider
- IMHO this should be de_DE.utf8 (without the "-") as "locale a" on command line tells.
Tested both variants yet, it seams UTF-8 and utf8 are handled same on my system.

2.) Do not work on Windows?
- I don't have windows, but as I read last days much about setlocale() for de_DE.utf8 this needs to be "German_Germany.65001" (or "deu_deu.65001").

On debian wheezy I have

C
C.UTF-8
de_DE
de_DE@euro
de_DE.iso88591
de_DE.iso885915@euro
de_DE.utf8
deutsch
dutch
français
french
fr_FR
fr_FR@euro
fr_FR.iso88591
fr_FR.iso885915@euro
fr_FR.utf8
german
nl_NL
nl_NL@euro
nl_NL.iso88591
nl_NL.iso885915@euro
nl_NL.utf8
POSIX

there are all kinds of variations. I would rather add another locale de_DE.utf8 . Not that familiar with tests, does skipping the test mean it won't be executed for second and third set from dataprovider if the first one fails?

No, only skip the test once, the other tests generated through dataProvider will be run.

My Ubuntu 15.04 returns:

C
C.UTF-8
de_AT.utf8
de_BE.utf8
de_CH.utf8
de_DE.utf8
de_LI.utf8
de_LU.utf8
en_AG
en_AG.utf8
en_AU.utf8
en_BW.utf8
en_CA.utf8
en_DK.utf8
en_GB.utf8
en_HK.utf8
en_IE.utf8
en_IN
en_IN.utf8
en_NG
en_NG.utf8
en_NZ.utf8
en_PH.utf8
en_SG.utf8
en_US.utf8
en_ZA.utf8
en_ZM
en_ZM.utf8
en_ZW.utf8
POSIX

But we don't need to add this as new runs in dataProvider, we can also add this as locale choose like array('de_DE.utf8', 'de_DE@euro', ... )