This was a stupid mistake I made I think, so I’m just going to quickly explain what happened here.
When I set up the menus for a website I’ve been working on in Joomla, I made a mistake and chose UK English instead of US English, so changing everything back into US English cost me some time. It was doable though. You have to first activate the target language in Language Manager-Content. Then based on C-A-M, I changed the language property of all the articles and categories one by one (I wish there was an easier way…). After that I changed language property for modules. Then I went back to language-content and disabled the language to be replaced, and switched on the target language.
However, my problem since then was that the language flag showing on the website was still the UK flag instead of US flag. It’s been bothering everybody, now I finally figured out why! Turns out you can’t just DECIDE for yourself what image prefix you will use for the content language! Instead, you have to go into the Image-language folder on FTP and see for yourself which prefix was used for the flag gif.
So I went into language-content and changed the image prefix code for US English. Hope the flag will show up soon.