Can confirm, I did some testing today using Vivaldi.
If I have Vivaldi's language list set to "en-us, en, de" it will use DE, if I add "es" at the end, it still uses DE, if I remove DE, I get ES. It doesn't matter what order it's in in the menu, it will always do this with these languages selected. The only time I can get it to load in English is if I have only English as the language in the selector box with no additional fallback languages.
Can confirm, I did some testing today using Vivaldi.
If I have Vivaldi's language list set to "en-us, en, de" it will use DE, if I add "es" at the end, it still uses DE, if I remove DE, I get ES. It doesn't matter what order it's in in the menu, it will always do this with these languages selected. The only time I can get it to load in English is if I have only English as the language in the selector box with no additional fallback languages.