The themeconfig.json file (your customized version of the siteconfig.json file) is used to configure your site for different languages.
You set the behaviour of the language selector in the toolbar, and define the different publications of a multi-language site, using the "filter" section of the siteconfig.json file. Copy this section into your themeconfig.json file and update, as described below.
To use the other elements in the themeconfig.json file to change your site in general, see: Site configuration.