Although you can build your course in any language, the Thinkific platform (including navigational buttons and menus) is only available in certain languages.
Currently, you can write your course in a right-to-left language (such as Arabic or Hebrew), however, these languages are not supported under Change Site Language Settings.
If you have a developer available and want to customize your site to hardcode your desired language, it would need to be done through the code on your site. We do not provide support for code customizations, however, we have some wonderful freelance web developers on our Thinkific Experts page who you can reach out to if you need them! There's also the Thinkific Store on Fiverr, for smaller, quicker jobs.
You can see more details on editing theme code here: Custom Themes (For Developers).
If you would like to add your vote to this feature request, feel free to contact Support!