tests: mark test as skippable if locale is not available

[Chris53897]

This tests fails for me locally, because i do not have the locale installed.
I have to run tests one by one instead of composer test
With this change it makes it easier to test local.

[frederikbosch]

But, if in CI, the test must fail if the locale is not there. So this solution is a little too easy.

[Chris53897]

ok, i will think about a more robust solution.

[Chris53897]

Is it valid to change it to a more commonly used Locale?
I guess US is comma seperated and the sense of the test ist still correct.
Please check this.

[frederikbosch]

I rather leave it as is.