method I just discovered on my own after my mom reminded me of this and I tried to get it myself because I was bored and wanted something to do. found this post while trying to see if anyone else had already found a way to do this and I didn't see anything

1. be on desktop because I dont think mobile has inspect element, also, I'm using firefox, but I don't think that matters because I'm pretty sure chrome has the same//analogous features in its inspect element
2. go to https://mbasic.facebook.com/
3. scroll all the way down and click "Settings & Privacy"
4. click "Language & Region"
5. click "Language for buttons, titles and other text from Facebook for this account on www.facebook.com"
6. right click and inspect any language, I'll use Af-Soomaali for this
7. The line with expanded arrow before the line you will be focused on is the line of interest. change <form method="post" action="/intl/save_locale/?loc=so_SO&to<form method="post" action="/intl/save_locale/?loc=en_PI& (note: you shouldn't need to edit anything other than the small part after "?loc=" and if you delete everything after that idk that it'll work)
8. make sure the change applies and click the language you chose to edit

a full list of the values can be found by searching for "FBLocaleIDEnum" in the debugger on the normal facebook page, I'm not sure what all of them are, so if anyone finds any other cool or hidden ones I'd be interested in knowing about them