Qt: Add note about relaunch Wireshark to apply change about language

Change-Id: Ibdc7d6b664d681de990608d689170b02439f0ae7
Reviewed-on: https://code.wireshark.org/review/3559
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>