Actually, in the case of many cars, it
is the same software, they just don't have the database of streets needed for the GPS to work.
So in the car case you pay more to have access to the streets database so you can use the GPS. In the computer case you pay more to have access to the language packs so you can have multiple languages.
The alternative to "people pay less when they don't need all the features" is "everyone pays the same even when they don't need all the features."
Do you want to pay the same amount when there are features you won't ever use? Or would you rather be able to choose to pay less when you don't want all the features?
You're thinking of it as "pay more to unlock features" when you should be thinking of it as "pay less to not have features you won't use."
Any additional programmer time to "lock" the features would be rather negligible next to the time the feature took in the first place.
Keep in mind also that applications not included with the system (which I suspect is the majority of applications people use on a daily basis) can be in whatever language they were written in and are not limited to the system language.
Edit: Of course, it's all kind of a historical point anyway since Windows 7 isn't "current" anymore and Windows 8 only came in two editions available to end-users. According to the
comparison chart, the language thing in particular is not differentiated between the SKUs, and other resources online seem to suggest that the non-Pro SKU also has support for multiple display languages. It's almost like Microsoft heard your feedback and did what you asked...but of course that would never happen...