Excel really is an amazing example of how too much localization is worse than none at all. It makes a lot of things harder for itself through the MS Office philosophy of "do everything wrong without asking", but it goes above and beyond that doing some genuinely insane things that nobody else even attempts.
In Excel, locale determines not just UI elements, but also:
Separators, like whether to use . or , for decimals (e.g. "1,000,000.00" becomes "1 000 000,00" in French)
The syntax of function delimiters and format specifiers, because these were originally chosen under the assumption that the first thing followed English-region rules
Date formats (e.g. US 10/31/2023 becomes 31/10/2023 in other locales). This (as well as differences in month and day-of-week names, etc.) influences what Excel automatically converts to a date when you don't want it to, which in turn influences what data is safe to enter manually.
Hotkeys: These are all different by locale, mostly for no real reason. For example in English "fill right" is Ctrl+R and "fill down" is Ctrl+D, but in Spanish Ctrl+D is to fill right (for derecha) and to fill down you use Ctrl+J (for abajo, yes really). These also have the distinction of being esoterically unknowable: there is no way to list all hotkeys within the app, and the official localized documentation only contains a translation of the English-region bindings. Better see if you can find one of those 500-page "How to use Office 2013" tomes at a library sale!
The names of most functions: e.g. BITRSHIFT becomes BIT.PRZESUNIĘCIE.W.PRAWO in Polish. (By the way, did you know that Windows treats AltGr as bidirectionally equivalent to Ctrl+Alt? Have fun with that!)
It need hardly be said that none of this is very well tested so there are 50x as many bugs in non-English locales, but the bigger problem is that none of this is at all configurable. You can't set a locale for an Excel file, or for Excel as a whole; it infers your locale from the operating system, and given how much of it simply doesn't work reliably, the standard workaround in all cases is to set your operating system locale to an English one. I've seen software that required you to change your region to the one it was made for, but Excel is the only thing I've ever seen that's internationalized so badly that it forces you to use all your other software in English. You'd think the move to a webapp version would fix this, right? But they did all they could to preserve the problem: now it infers your region from your OneDrive settings rather than your OS.
Of course, Excel tries to automatically tweak all this to match your region when you open a file, and of course it's not smart enough to always do it right, but the key thing to note is that it makes these tweaks by destructive changes to the file. They look superficially non-destructive because it will change them back if you open them in another region, but the conversion isn't reliable enough to be 100% safe, and if you were hoping to collaborate simultaneously on the same file with someone working internationally, or view it while they have a lock, go straight to hell.
I know complaining about Excel is some 1990 Dilbert kind of shit: MS Office as a whole is a kind of cautionary fable about how it's better for software to be consistent than clever. But I avoided this rabbithole for a long time and so I haven't yet got over my astonishment at how much work they put into creating problems that no other software has, only to lead to a situation where the ultimate result is "avoid using Excel on any computer set to a non-English region, if at all possible."
