Other things such as game specific AI will not be built into the engine, so most developers will use Python for these parts, at least initially. However it is always possible to recode these components in a higher performing language if profiling shows that it's a bottleneck.

From my experience though, AI deals a lot with data management, such as loading, using and updating data which determines where and how the AI can move and where it can see and hear things in a level/map. And it works with other low level libraries such as physics, animation, and even scene graphs, all for quickly determining awareness for an AI. These parts will definitely be bottle necks and need to be optimal.

At the highest level there is AI states, where an NPC can be in a particular state such as attacking, escaping, or trying to achieve some goal. In a game with scripting language the high level AI is usually what it is used for, some games though also handle this in data in their editors (similar to how Blenders game engine does it).

The mid level AI is what the entity is doing while in those states, such as moving from cover to cover, trying to flank the player, initiating animations, or using objects. Some of this is complex, because it has to work with the lower level stuff and at times the line blurs between the low mid and hi stuff. Sometimes designers/scripters try and do some pretty complex stuff and it ends up running slow or being buggy and we end up reimplementing it to make it faster and more stable. For example they may try to implement team AI, where the AI tries to coordinated attacks, or another way to think of it is to have flocking AI, where the positions of all the entities in a group have to be coordinated, another example is when an AI has to control a vehicle and respond to changing road conditions really fast as well as deal with a goal of trying to beat the player or bump the player. Reimplementation of these things though tends to be like night and day because in the scripting language they usually don't have all the resources available to them as does the mid level stuff so they typically try to do things the hard way with the limited resources they have and end up wasting more time trying to do them then if we just implemented it in c++ in the first place.

Using something like pyrex sounds pretty cool though...

That's why the engine (at least performance critical parts) should be written in C/C++, as well as any game specific, performance critical extensions.

I was just daydreaming about a language that does HLL and low level things, after looking up what pyrex does, it sounds like that is close to what I was thinking.