I'm no hydro specialist but my understanding is that in the Alps/Pyrénées, the hydro capacity is essentially maxed out because any additional projects will be denied on environmental grounds (flooding valleys is, as it turns out, not amazing for the environment). Maybe there's some pairs of existing artificial lakes that could act as batteries, IDK. But I wouldn't expect this to magically solve every issue.
In Belgium we have one such installation thanks to a fortuitous topological quirk, but no plans for more. Still, 10 GWh is not nothing and already helping a lot to absorb renewable fluctuations.