Is it possible to get decaf coffee in Paris/Bruge?
I can't vouch for Paris, but decaf is available in Belgium. Of course, that doesn't necessarily imply that any given restaurant serves it.
Yes, in Paris its that instant stuff often, Nescafe is widely available. Herbal teas are very common if you can't find decaf coffee, they are called Tisanes.
Below the "fine-restaurant" level, it is common in France and Belgium for decaf to be available only as powdered instant. As others noted, it IS widely available. Since many better restaurants use an Espresso machine, it's easier for them to make decaf drip/espresso if they have the grounds on hand. But again, it is less common than in the US.