Hello there,
Can you send me a screenshot with this error?
If you actually change the default language (something that’s perfectly fine on a brand new site without content), you’ll need to change all the content you’ve written to the new language and then translate again all the information. There is no support to automate this as translations are directional. So yes, you’ll lose your translations and also your content will not make sense, as it’s expected to be written in the default language.
However, you can change the first shown language (not the default language in which the content is written in) by simply going to TranslatePress Settings -> Enable Subdirectory for Default Language, then reorder the languages by drag and drop. Check our FAQ page about this topic.
Best Regards,
Anca