Apple pie a la mode is an American idiom adapted from its French roots. It is not French and does not follow, nor does it need to follow, the strict rules of French grammar.
Two questions:
1. Do you understand what an idiom is? By definition idioms and idiomatic expressions stand outside the normal grammatical rules.
2. How many years have you been a putatively sentient being here in the US without knowing exactly what apple pie a la mode meant?
You have got to be fucking kidding me.
Seriously?
You want to bring that elitist approach to every discussion possible?
I know what idioms are. I've had to qualify idioms to people who miss the point of what the idiom even means, versus dissecting the literal roots. And they make learning other languages more challenging; translating English idioms to Russian, and then even understanding Russian idioms... fun times.
I've known my entire life that a la mode means with ice cream, at least in America and in regards to desserts. Um... apple pie is one of my favorite fall desserts, especially when coupled with ice cream.
How did you even gleam
any of that nonsense from what I typed.
And technically, it does indeed translate perfectly, you just have to know what it means to have apple pie served in the popular/trendy style. It's assuming you know what the style is, and while it may technically be an idiom since it does not have English roots and is used without translation, it's not the typical idiom.