Don’t know if there is an app but I think in France there are many restaurants which are refilling your bottle for free. Usually they have some sticker on their entrance.
In Switzerland you can refill your bottle from many fountains (as long as there is not written “KEIN TRINKWASSER”). In Italy there are also many fountains in the city.
Best to ask locals (although many are buying plastic bottles saying that the tap water has bad taste, so don’t count always on their opinion).

Restaurants refilling for free without any purchase?! I wouldn’t like to ask

See here: https://www.trendingtopics.eu/plastic-free-shops-in-paris-fill-up-drinking-bottles-for-free/

Just fill your bottle at the taps and sinks in the toilets of the train stations or any other place? It might cost some eurocents, but still much less than 2,70 EUR for a plastic bottle of water ;)

If it’s plus +30°C people really won’t look weirdly if you ask to fill up your gourd or water bottle.

Well, in some toilets it might be a bit disgusting when I see people washing there feets etc. there in the washbasin :-D But in a restaurant I wouldn’t see a problem.