There is an ancient structure in Finnish and to my limited knowledge generally in Finno-Ugric languages, that has to do with hunting, picking berries and similar activities. You use the location cases to show such action:
- Olen metsällä : (lit.) "I am on the forest" meaning "I am hunting"
- Olen hirvessä : (lit.) "I am in a/the elk" meaning "I am somewhere hunting elks"
- Olemme Pyhäjärvellä kalassa : (lit.) "We are… in a/the fish" meaning "We are on the lake Pyhäjärvi fishing"
- Olen marjassa : (lit.) "I am in a/the berry" meaning "I am somewhere picking berries"
- Oletko mustikassa? : (lit.) "Are you in a/the blueberry?" meaning "Are you somewhere picking blueberries?"
- Olen Salossa sienessä : (lit.) "I am… in a/the mushroom" meaning "I am in (the municipality) Salo picking mushrooms"
Later this structure has found use for instance in games, websites etc. not least because of the influence of Swedish and English:
- Oletko Duolingossa? : Are you in Duolingo? = Are you using Duolingo?
- Olemme Facebookissa : We are in Facebook = We are using Facebook.
Fascinating how French shares this structure in some ways, for example I would say "J'suis aux mûres"- "I am at the blackberries" for when I am in the nature picking them, same for "aux champignons" "at the mushrooms", "aux châtaignes" "at the chestnuts", but strangely we share many other uncommon features, like saying something that means I have [ ] to say I am cold/hungry/thirst etc, but also partitive somehow
Indeed. The Indoeuropean languages (IE) used to have more grammatical cases, but many of those have been replaced by prepositions or are otherwise no longer used. To my knowledge Russian has retained the partitive case ("Он съел хлеба"*) and French has replaced it directly with a prepositional expression ("J’ai mangé du pain"). However the Germanic languages seem to have replaced it completely with other expressions ("some bread/ein bisschen Brot/lite bröd…").
It may well be the same with this kind of use of location to show action in the IE languages: it has been there but most have dropped it during the millenia. French has retained it.
*: You guessed right, leipä is a loan from Russian (← хлеб).