Lacking options for both audio and subtitles

Buy/Rent movies get the audio language tracks and subtitle tracks as decided on by the supplying studios, which may differ by country (or country-like region), and may differ by title. Apple doesn’t mandate any of this.

The Apple TV app on the Apple TV box (or other Apple devices) may also show subscription options for the same title from other content providers, which may have their own audio tracks and subtitle tracks, that might differ from what is offered through the Store in the Apple TV app.

There is no support for manually adding audio or subtitle tracks. You’ll have to select from what is offered.

If you see better options elsewhere, and if you think that outweighs splitting up your collection among various content providers, then feel free to get your content where you think it is best. Or try contacting the originating studio with your grievance about this, and for them to do better for future titles.

I’m a US citizen that lives in Spain, my girlfriend is Spanish and we like to switch back and fourth on audio and subtitle options in both languages depending on the show or movie.

What matters is the country as set for the Apple ID account; that determines in what region of the Store you are shopping. The Spain Store may not always have English options for many titles, although some studios are more generous than others. The US Store will often have Spanish options for various titles, as there is a considerable (~12.5%) (Latin American) Spanish speaking population there.

I understand that offering extra tracks is a real extra expense for studios, even if these tracks already exist, as royalties will have to be paid by market exposure/​sales. So some studios can be cheap about that, and minimize to what they think is enough for most consumers, per market region.