Fix translations and UpdatedVersionAvailableUserAlert.updateNowButton

[ArneBab]

It used the FALLBACK_LANG to get the fallback lang, but iterator().next() skipped to the next value, which is the raw key.

This had also broken our whole translation system.

[vwoodzell]

I agree with the change to MainJarUpdater. I can't imagine why we'd want to show the fallback string there instead of the localized one.

I don't think that BaseL10n.getDefaultString() should be changed, though. It doesn't return the raw key unless the key is missing from the fallback translation. I've added tests that show it working correctly.

[ArneBab]

Thank you for checking! I merged your tests and reverted the change to getDefaultString(). Also I changed the return value documentation to match the implementation.

Good to go?