You need to start the PicApport server with the additional parameter -Duser.language=en
If you are using our Debian package, you need to modify the file /opt/picapport/StartPicApport.sh add the parameter after the java command. For example:
java -Duser.language=en -DTRACE=Error -Duser.home=/opt/picapport -Xmx512m -jar picapport-headless.jar -pclient.timeout.short.millis=15000