When you enter the location of a job, WP Job Manager will fetch data about this location from Google. This isn’t used by default, but some themes/add-ons use it to display jobs on maps etc.
This geolocation data will default to the site’s language selected in Settings -> General.
To specify another language to be used for geolocation, you can add the following code to your theme’s functions.php file, and replace the “en_GB” with the code for your chosen language:
You can find available language codes here.