Use locales with the customary names.
On glibc systems, nowadays, the German and French locales are named
'de_DE.UTF-8' and 'fr_FR.UTF-8', respectively. This change avoids the need
to install similar locales with different names.
* gettext-tools/tests/gettext-3: Use locales with '.UTF-8' suffix.
* gettext-tools/tests/gettext-3-prg.c (lang): Likewise.
