If you click "Minecraft Realms" with the Polish language set to, you will get the information "Przestarzały klient! Twój klient jest przestarzały i niekompatybilny z Realms. Proszę zaktualizować Minecrafta do najnowszej wersji. " English: "Client Outdated! Your client is outdated and not compatible with Realms. Please update to the most recent version of Minecraft." And it should be "Niekompatybilny klient! Realms jest niekompatybilny ze snapshotami." English: " Client Incompatible! Realms is not compatible with snapshot versions."

This is not a feature request.

How to fix it:
-in snapshots, change "mco.client.outdated.title" to "mco.client.incompatible.title" and "mco.client.outdated.msg.line1" to "mco.client.incompatible.msg.line3" and remove "mco.client.outdated.msg.line2".