A It's true that in some parts of Germany, in some vintages, the wines are too acidic. To cope with this, water may be added, or sugar, or the winemaker could try his luck with malolactic fermentation to take away the unwanted sharpness.
Chalk - calcium carbonate - can be a very useful tool, though its use is subject to regulation and can only be employed with a permit. It turns unwanted acidity into tartrates which settle to the bottom of the tank and can be filtered out. Some carbon dioxide is also generated in the process, which may not be the best friend of the environment but is totally harmless as far as the wine is concerned. So your Riesling won't taste chalky.
The addition of calcium carbonate is not restricted to Germany. English winemakers occasionally use it, as do their counterparts in Luxembourg, Canada, Tasmania and parts of New Zealand.