When driving in Belgium, what equipment is mandatory to have in a car registered in another country?

They are not mandatory for foreign cars.
My parents live in Belgium and I live just across the border in the Netherlands.
I visit my parents almost daily, by car, so I had reason to check this myself.

I checked the Belgian traffic code:
This indicates clearly that the extinguisher and the first-aid kit are considered to be “part of the vehicle” and therefore only apply to vehicles that have a Belgian registration. (They must have the kit in order to be considered road-legal.) This is not enforceable on foreign cars: Because of international traffic regulations Belgium is required to allow road-legal cars from other nations on their roads as long as they are reasonably safe to drive. The presence (or absence) of an add-on not essential to the operation of the car (like a first-aid kit) can’t be part of any restriction that prohibits foreign cars on Belgian roads.
The triangular sign and the vest are mandatory to use in case of an accident, car-trouble or other dangerous situation. If you don’t have them in the car you are not in trouble, unless you get in a situation where you are required to actually use them.
If you don’t have a vest and need to leave the car on the middle lane of a 3-lane motorway the policy-officer that shows up, will fine you for not having one. (Happened to my cousin when his car got in a fender-bender on the Brussels ring-road.)

I also asked my parents neighbor who is a Belgian police officer:
He confirmed everything above too. He also mentioned that the fire-extinguisher needs to be a certified model and not beyond its expiration date either.
This is effectively never checked, unless the police officer is really looking for a reason to hassle you.
(He hazarded a guess that at least 90% of Belgians drive around with an expired, or empty, extinguisher. The thing is only in the car to satisfy a cursory glance of the police.)

According to a page on the site of (in Dutch) the Royal Dutch Touring Club, you’re required to have a warning triangle and a safety vest. While a fire extinguisher and first aid kit are only required for Belgian drivers, they still recommend taking them as well. Another page of theirs (in Dutch) which gives an overview of these rules for various European countries says that local police does not always take into account that some rules don’t apply to cars registered in foreign countries.

Legally speaking there is a difference between rules that apply to the car and rules that apply to the behavior of the driver. The requirement of having a fire extinguisher and first aid kit are legally specified as rules that apply to the car, and therefore only apply to cars registered in Belgium. As for the warning triangle and safety vest, according to a discussion in the “nl.auto” newsgroup (in Dutch), the Belgian law does not state that these are required to be present in a car, like the other two items, but instead specifies that when leaving the car in the middle of the road, you need to be wearing a safety vest. If you have an accident, you’re required to put out a safety triangle. So you’re not technically required to have these items in the car, but it would be difficult to follow the law otherwise. For the safety vest this was quite literally explained in a text accompanying the relevant Royal Order:

La modification du Code de la Route ne s’accompagne pas d’une adaptation de la réglementation technique des véhicules. En effet, si la veste réfléchissante était imposée comme équipement obligatoire du véhicule, seuls ceux immatriculés en Belgique seraient concernés. Il est donc plus logique d’introduire cette obligation sous la forme d’une règle de comportement même si l’utilisation de cette veste suppose nécessairement que le conducteur en dispose d’une dans son véhicule.

Translated, this says:

The modification of the highway code is not accompanied by an adaptation of the technical regulations for vehicles. Should the safety vest be imposed as an obligatory equipment of the vehicle, then this regulation would only apply to vehicles registered in Belgium. It therefore makes more sense to introduce this obligation as a rule of behavior, although the use of the safety vest necessarily implies that the driver has one in his vehicle.

Edit: It has been pointed out to me that Belgian law does also have a rule for safety vests which applies to the car. This is the result of a Royal Order that did introduce an adaptation of the technical regulations for vehicles. It was issued about 2 years after the above one, in 2009. As they stand, there is now a behavioral rule which applies to anyone driving in Belgium, and a car equipment rule which applies to any car registered in Belgium. There is no accompanying text for the 2009 Royal Order which explains why the extra rule was introduced after all. I suspect the first one did not give enough legal basis to fine people for not having the safety vest in the car, as it only “implied” you needed to have one. As far as I know, the requirement of having a fire extinguisher and first aid kit are (still) only specified as rules that apply to the car.

However, in summary, rather than trying to argue the finer points of these legal distinctions with local police, I would recommend following the advice of the Royal Dutch Touring Club by simply taking all four items. It avoids the need for discussion, the possibility of fines, and they might be useful if you do have an accident.

The Dutch and British automobile clubs both suggest that fire extinguishers and first-aid kits are only compulsory for cars registered locally as do the ACL from Luxembourg. Not necessarily authoritative but I think they would know.

For some reason, the “service public fédéral mobilité” (that’s the name of the federal ministry for transport) has nothing to say about this and only mentions safety vests, which is not particularly helpful…

It seems that the basis for all these rules is the règlement technique des véhicules automobiles (in particular articles 70 and 71). Article 2 provides that

Sont soumis aux prescriptions du présent règlement général, les véhicules automobiles circulant sous couvert d’une plaque d’immatriculation belge, ainsi que les remorques belges tirées par eux.

This would seem to be authoritative and to confirm that foreign-registered cars are exempt from all these requirements (but it’s always dangerous to try to interpret a legal text when you know nothing about the legal system…).

A first-aid kit is quite cheap and recommended or mandatory in many places so you could probably consider getting one even if a fire extinguisher seems too much of a hassle.

Finally, and keeping in mind that such anecdotes are not worth much, I have had many occasions to drive in Belgium and have never experienced or witnessed a police check on the road so I would not worry too much about it.

Although the Belgian road code does mention triangle, estinguisher, first-aid kit and high-visibility jacket (see articles 70-70bis-71 here), according to the Bison Fute governmental website, you must have:

• 1x Triangle
• 1x high-visibility vest per passenger
• 1x Fire extinguisher

Quoting from the website:

Equipement obligatoire à bord du véhicule

• 1 triangle de signalisation
• 1 gilet par occupant du véhicule
• 1 extincteur

This seems to agree with this other French website on the topic. Nowhere does either website mention a first-aid kit.

To be extra sure I would follow the advice on the Bison Fute website, and I would ask the first police officer you meet:

Ces informations sont données à titre indicatif et ne sont pas exhaustives. Il est conseillé de s’informer au passage à la frontière sur les autres réglementations en vigueur.