Do you have a water source to easily re-fill your pond? Is it already full of fish?

How steep are your maximum slopes?. In moist clay you can work in the bentonite and re-shape the slope at pretty steep angles.

If you have the option of draining the pond and letting it mostly dry, then you can apply your bentonite on the pond slopes and the bottom and then roto-till about 8-10 cm deep to get uniform mixing. Tamp or lightly compact the entire pond area after tilling.

Bentonite is a type of clay mineral that actually incorporates water into the crystalline structure. It therefore swells substantially after it is hydrated. However, it is a little fragile as regards staying in place until it has swelled. Therefore you need to gently re-fill the pond from the bottom up such that you do NOT create any erosion channels that cut through your 10 cm blanket.

The treatment above should create a pretty good seal for a pond that is only 2m deep.

Bentonite may not be a good solution if the water levels in your pond vary over extended periods of time. If your bentonite blanket dries out, then it may suffer from large cracks as it dehydrates. Re-filling the pond may not perfectly heal these cracks.

If you can't drain and treat your pond, then bentonite can be added into a full pond. However, TJ has a good product that may be a better solution for that type of application or for the situation where your pond goes through wet/dry cycles.

I have no idea about the regulations or cost to get that product to Sweden. However, TJ is a "sealing expert" and can also just give you solid advice if you can contact him, or if he shows up in your thread.

Good luck on your new pond project!