i18n: add translations

The pipewire.pot file is made with 'meson compile pipewire-pot' by
scanning the files in POTFILES.in.
The .po files were generated with 'meson compile pipewire-update-po',
on .po file per entry in LINGUAS.
The .po files were filled with:
'msgmerge -N $pulseaudio/po/$lang.po po/$lang.po -o po/$lang.po' to
get a decent set of translations for all the ACP strings.

Run 'LANGUAGE=nl make run' to start the server in a language,
test with 'LANGUAGE=nl pavucontrol'