For some reason the game defaults to Brazilian Portuguese, which is my language but I wanted to play in English.

The LanguageSelector app that came with the game is set to English, but the game refuses to acknowledge it.

What's even funnier is that my Windows itself is in English so I have no idea how or why the game defaults to pt-br and most importantly why I'm unable to change it.

After googling around I found a very old Reddit post with instructions to change a .mxml file in the Settings folder, but its language parameters are already set to English.

There was supposed to be an option to change the game's language in the Galaxy client's "Manage Installation" > "Configure..." menu, but it's grayed out and says "Managed in-game only".

I also found a rather recent post here about the Fractals update and language changes not working for the GOG release. I have no idea if that is still the case.

If there is a solution, any assistance is appreciated.

They added a ingame option to change the language, might give that a try.
Maybe it is somehow overwriting your language selector app choice.
Keep in mind that you might have to restart the game after changing the language ingame.

Thanks. I missed that option because it was labeled as "improper language" in Portuguese. I thought it was some sort of profanity filter for the ingame chat.

All the more reason to swap to English. What an odd localization error...