System language was automatically set in English.

And yes, having an english dashboard makes a difference. Games automatically launch in the language the dashboard is set to and it is not possible to change the language inside the game.

If I launch Halo, all the voices will be in spanish if i have my region set to spain. Once I change that to UK, the language of the game automatically changes to English

Have you set the language to English in Settings > System Settings > Language and Location > Language > English?

If so, there's not much you can do besides that.

If you go to the console settings you can change the language to English go to:

System settings > Console settings > language and locale > Choose English.

Ok thank you, that actually solved my problem!

The game apparently defaults to French when the language is set to English (and only French and German are available languages for the game).

My region was set to Germany for some time but my console was always set to English. And the game always defaulted to French. So the game doesn't care about my region settings and only checks the language settings. When changing the console/system language to German the game is also German (English is not an option and when set as the system language changes the games language to French).

...the game has to have the option INSIDE the game, isnt it?

There are multiple factors that might or might not allow you to change a language.

First of all: The game has to have the specific language installed. Depending on the game, some games ONLY have one language built in (even if it's available in different languages around the world). If that is such a game, there is nothing you can do.

If the game DOES HAVE the different languages (and English is usually always present, if it's a multi-language installation), then it depends on how the game chooses the language.

Most modern games (and games that come from consoles) try to detect the language from the settings of the Operating System.

So if you run your Windows 10 in Spanish, the game will automatically be in Spanish.

Some games do have settings to override that automatic setting, but not all games do have that option. If the game does not have the option to change it, and you are SURE that the game has the English language pack installed, then you need to change the
language of your Operating System.