If you have a system with systemd you can start seafile and seahub via unit files. In the unit file for seahub you can set the environment and the message is gone:
[Unit]
Description=Seafile hub
After=network.target seafile.service
[Service]
# change start to start-fastcgi if you want to run fastcgi
Environment="LC_ALL=C"
ExecStart=/home/seafile/seafile-server-latest/seahub.sh start-fastcgi
ExecStop=/home/seafile/seafile-server-latest/seahub.sh stop
User=seafile
Group=seafile
Type=oneshot
RemainAfterExit=yes
[Install]
WantedBy=multi-user.target