Install pology and run the source/tools/l10n/updateTemplates.py script. Notice that the new strings (should) show up in the transifex pology files.
If you fail to install pology because there are no repositories available, compare this piece of code against the other translated strings, and in case this is committed, check the next autobuild committed pology files to verify.
To test that the strings are actually displayed,
change in `default.cfg`:
server = "lobby2.wildfiregames.com" ; Address of lobby server
and in XmppClient::onDisconnect
CreateGUIMessage("system", "disconnected", std::time(nullptr), "reason", TLSErrorToString(gloox::CertWrongPeer));
for instance.