[Resolved] Is it possible to change the "main" language.

If you are translating content by yourself, the Advanced Translation Editor is WPML's default translation editor and recommended way of translating content. If you previously translated content using the Classic Translation Editor, we recommend that you continue editing those translations with the Classic Translation Editor. To translate content from plugins that are not compatible with WPML, you need to use manual translation.
Once a piece of content is translated with a specific method, you should stick to that method, otherwise, the translations will not remain in sync.

If needed, you can also change the main, otherwise known as default language of your site by going to WPML > Languages. When you change your site’s default language, the new default language becomes your site’s main language. It’s the first language users see when they visit your website. WPML also automatically adjusts your content and links to point to the corresponding language.