Not too long ago, LG came out with a press release stating that they would be the first to roll out Android 6.0 Marshmallow to a non-Nexus device. Now that wasn't too surprising, since they did the same thing last year. And it was also started in Poland. The LG G4 in Poland was due to start its Marshmallow update first, however it has been paused due to some "technical reasons". As is normal with these things, LG has not given out any details about why the update was paused. It could be due to some bugs, or some other reason. It's tough to say, however, it should be back on track real soon.
LG confirmed on Facebook that the update was halted, in a comment to a customer asking where the update was. Citing "technical reasons" and also saying they'll keep their customers informed. This isn't surprising because the update was rather quick, and likely rushed out. Which means there could be a slew of bugs in the update, and that is likely what's affecting the update to the G4 users in Poland.
However, if you are a G4 owner and have the H815 model, you can still download the firmware and flash it onto your device, using LG's Mobile tool. The KDZ is available over on XDA for your flashing needs. It's important to remember that this KDZ file is only for the H815 model, and you will also need to be rooted and have an unlocked bootloader for this to be flashed. As always, you should take all responsibility for flashing this KDZ, in case something catastrophic happens with your LG G4. If you're not comfortable with flashing the image, then I would suggest you wait for the OTA to commence again and hit your device. That way you don't risk bricking your LG G4 or something worse.
Marshmallow is rolling out to all current Nexus devices now and Motorola is likely going to be next (they are already testing Marshmallow on the Moto X Pure Edition). And with LG working on rolling out Marshmallow to Poland now, it could be very soon that we see it available for other regions.