Region-specific localizations do not work on some versions of Linux

[tcobbs-bentley]

Describe the bug
The CI build machines apparently recently updated their Linux OS in a way that causes @POSIX to be appended to their region-specific language names. (For example: en-US@POSIX.) This cause our i18n implementation to fail to load those region-specific languages. This originally caused all CI jobs to fail, since there is a test to verify the presence of en and en-US translations, but a workaround was committed skip that failing test when running on Linux.

To Reproduce
Steps to reproduce the behavior:

1. Run iTwin on an operating system that appends @POSIX to the region-specific language name.
2. Include both en and en-US translations.
3. Observe that the en translation is used instead of the en-US translation.

Expected behavior
Region-specific translations should work in the presence of the @POSIX suffix on language names.

• OS: Linux