The regional lects are still widely spoken in Slovenia, Slovakia, Croatia, Serbia and Bulgaria at least.
Slovakia still has up to 10 large regional dialects, and there appear to still be city dialects within those, so dialects are going strong in Slovakia.
Dialects are still very strong in Slovenia, with 10 major ones that are often quite divergent. Prekmurjian is actually a separate language.
Dialects are also going strong in Croatia, and in some places there is nearly a different dialect in every town, especially on the Adriatic coast. In the center-north there are also many regional dialects and even cases of one in every town or city. The east is fairly uniform dialectally, and it resembles the Serbian across the border. Kaykavian and Chakavian, instead of being Croatian dialects, are separate languages. Molise Croatian in Italy is not a dialect either; instead it is a separate language. To the north, there is Burgenland Croatian, many dialects of which form a separate language.
Serbia still has major regional dialects, but I doubt if it is to the point of even a different one in every major city. Torlakian in the south is a separate language, with poor intelligibility of dialects in the north like Sumaija.
Bulgaria is still wildly diverse dialectally, with maybe 10 major dialect divisions. In the far West, you definitely run into the dialect in every town situation. The far west and far east on the coast of the Black Sea cannot understand each other. In the far West, the dialects are part of a separate language called Torlakian.
I know nothing about the dialect situation in Hungary or Greece, though Greek has 3-4 languages inside of it.
Albania has 5-6 languages inside of it, but four of those are spoken in Italy. Inside Albania, there are two major lects, Tosk and Gheg, and it is quite controversial how well they understand each other.
Macedonia is wildly diverse dialectally, heading towards the dialect in every town scenario, but apparently most can understand each other. Kumanovo is actually part of a separate language called Torlakian.
Czechoslovakia used to have many dialects, but the regional dialects are dying out. Lach is a dialect which has gone all the way to a separate language; in fact, it is two separate languages, as Eastern Lach and Western Lach cannot understand each other. Ciezsin Silesian is a separate language between Polish and Czech, but it is more a part of Czech than a part of Polish.
Poland once had dialects too, but those seem to be mostly dying out. The so-called Polish “dialects” Kashubian and Silesian are actually completely separate languages.
There are definitely still regional dialects in Ukraine, maybe 10-15. The lects in the far West, Western Ukrainian and far East, Eastern Ukrainian, are arguably separate languages and cannot understand each other.
There are not many regional dialects in Romania, and they can all understand each other.
There are some dialects in Belarus.
There are not many regional dialects in European Russia.
The dialect situation is intensified in Germany, the Netherlands, France, and parts of Spain, where you can have a different regional lect in nearly every city if not every town!
So Spain will recognize Catalan, Basque, Galician and Aranese as official languages, since Catalan is about as French as it is Spanish. Aranese is Occitan, and no one thinks Occitan is Spanish. If anything, it and Catalan form a separate group maybe called Catalan-Occitan. Basque is obviously not Spanish at all. Galician is tricky, but really it’s more Portuguese than Spanish. In fact, a good argument can be made that Portuguese is a dialect of Galician instead of the other way around.
However, Spain, like all these countries above, is terrified of making official any of the lects that look anything like the official language, which is Spanish. So Asturian-Leonese, Extremaduran-Cantabrian, Aragonese are all denied official recognition on the grounds that they are all “Castilian dialects.”
Actually they are not, and honestly, you can make a better argument that Castilian is a dialect of Asturian-Leonese, or even better of Extremaduran-Cantabrian, as Castilian grew out of the Cantabrian region about 1,000 years ago. Asturias, Leon, and Aragon for many centuries were not even under Spanish rule but instead were under various crowns such as the Crown of Leon, the Crown of Asturias, and the Crown of Aragon. That’s why these Macro-Spanish lects are still separate languages – because they were under these separate crowns for so long, so they grew quite distinctive.
I think there are maybe 1,000 second language speakers of Cornish. The Cornish revival ran into problems due to a typical idiotic fight over writing standards. There are no native speakers forever now, and it hadn’t been written for 300 years or maybe ever, and there are maybe 1,000 native speakers at most. So what did they do? They got into a huge drawn-out fight over which of two writing standards to use! The community split into two with about half defending one and half defending the other. Idiotic!
I am not sure how many new Manx speakers there are. Some figures said there were up to 2,500 new speakers who have some command of the language. Some of these second language speakers have learned it quite well, and some couples are raising their small children in Manx! So now we have some Manx native speakers showing up at school at age five. It’s pretty incredible. There are also some education programs going on.
Cornish is closer to Breton than to Scottish Gaelic and Irish Gaelic. Cornish, Welsh, and Breton form one group in Gaelic and Irish Gaelic, Scottish Gaelic and Manx form another.
Some people think that Cornish and Breton are mutually intelligible, but that does not seem to be the case. There are reports from the old days of Cornish speakers going over to Brittany, and while they both recognized they were speaking a similar language, they could not communicate much with each other. But a speaker of one could probably pick up the other quite quickly.
Some say Irish Gaelic and Scottish Gaelic are mutually intelligible, but I doubt if that’s true. It’s probably more the case of ~75% mutual intelligibility, high but not full. But they can carry on talking to each other reasonably well.
