Change language of games on xbox beta without changing my windows region?

I think the language in Windows 10 Store games and apps depends on region setting, not system language.

Nope. The region setting only changes the type of currency, I've already tried it, it does not change the language of the game, neither the system language does. I've already uninstalled the game, changed my system language to english, reinstalled the game,
and it did nothing, the game installed in spanish either way. I'm suspicious it has to do with your Xbox account region, I'm going to try that next.

This would be better placed in the feedback forums here:
forums.xbox.com/.../default.aspx

That's not changing your region or language. That's your locale on your console. The only way to change the account region language would be by going to the link I gave you above and following the directions under Change your Xbox LIVE account language.
You followed the directions for Change your Xbox 360 console language.

The console language does not affect the language of the game it's only for family settings and WiFi settings if you use those two things. The Xbox LIVE account language is what's going to affect the language of the game and your account.

But as I said, ultimately it's up to the developer to make it so there are multiple language options from within the games menu. Without those two options available to you, you will not be able to change the language of a.) your account (if there's no preferred
language drop down) or b.) change the language of the game (if there's no settings to change the game language).

Unless the game specifically allows language switching in it's options menu (not all games have a language switching option), and you are SURE the game has other languages included, you might have to change your Windows display language.

Open the Windows settings by clicking the start menu, and then clicking the gear icon. Or type "Settings" in the search bar.

In the settings go to "Time and Language". Then go to "Language". There change the Windows display language to "English". If necessary, have Windows download the language and reboot.

Then try to start the game again. If that didn't work, go to the Time and Language settings again, and under "Region" change the country or region to "English USA" or "English United Kingdom".

You cannot download 360 games with gold without an adult account or a 360

Xbox one parental settings don't apply to xbox 360 markrtplace

Xbox one games are region independent as far as games goes and language independent

For xbox 360 most games with gold are region locked your account region must reflect your actual region you can change this Under my account and change my region on this website

After that change it on your console

If you don't match your account region to your actual region you will continue to have issues with downloading xbox 360 games

And the language independence only applies to one games