gecko: check app compatibility for -i18n via package registry

make[5]: "/usr/ports/Mk/Uses/gecko.mk" line 48: warning: "/usr/local/bin/firefox --version 2>/dev/null" returned non-zero status

$ sudo firefox --version
Running Nightly as root in a regular user's session is not supported. ($XAUTHORITY is /home/foo/.Xauthority which is owned by foo.)

https://bugzilla.mozilla.org/show_bug.cgi?id=1323302

PR: 229064
Reported by: Vladimir Omelchuk
Analyzed by: Rainer Hurling
Submitted by: Walter Schwarzenfeld (based on)