I know. We haven't interconnected the entire planet. So it still depends on what region we are talking about. Even inside a country some regions might be barely connected (see the Texas power grid).
> might cause a coal power station to be started in Poland
It also might cause a natural gas station to be started up. Again, that depends on the location. Usually it would cause a natural gas power plant to start up. It could also be pumped storage hydro, or even batteries.
Since you are talking about lack of knowledge, it's useful to point out that coal is very unlikely to be used as a peaker plant, just like we don't start nuclear power stations for the same purpose. Generally they are used for baseline power.
We aren't going to be starting coal plants because France is using more power. Some countries might decide to build more though, to account for the predicted energy usage growth.
> This is highly location-dependent.
It really isn’t. Electricity moves through a network: for example using more electricity in France might cause a coal power station to be started in Poland. The same occurs for any network.
Additionally, when you add an EXTRA kWh of load, what really matters is where that extra kWh is generated. Think of it that all the electricity on the network is already being used (all the green electricity is virtually guaranteed to be used).
When you add one marginal kWh of usage, that kWh is often genersted 100% from gas or coal.
Obviously there are a lot of different situations where the above points are not relevant, and where an efficient coal station might be better than an ICE. I am just saying that many people are ignorant of how the electricity networks work.