The “language selection” dialog allows to switch the language. Beside the built-in language English there is a bunch of additional languages which will be search for in the directory
<installation prefix>/share/worker/catalogs/
(usually /usr/share… or /usr/local/share.. but that may differ on your system). Additionally Worker also looks in the catalog directory in the worker configuration directory
$HOME/.worker/catalogs
You can create additional translations and store them in that directory.