There are a wide range of actual languages (by the separate origin/lack of mutual intelligibility standard) spoken by ethnic minorities in most European countries.
People have already picked up on some of the classics.
For France, in addition to Breton and a number of Basque dialects, there are a small but significant number of Flemish speakers, the remaining speakers of Occitan (most of whom are fluent in langue-d’oil French as well), and some Savoyards who speak what’s considered a separate language from either French or Occitan. As of the older references I have, there were a lot of Alsatian speakers of a Hochdeutsch dialect as well; whether this remains true, I’d have to defer to someone familiar with the area.
Belgium, in addition to being split between Flemish and Walloon French, also has about a 2% German-speaking minority (concentrated in the Eupen/Malmedy area).
Germany, with still a fair number of bilingual Plattdeutsch speakers, also has an ethnic minority in Saxony (former southern East Germany) of Slavic speakers of Sorbian or Wendish (both names valid; which is preferred, I don’t know.)
Switzerland is largely Schwyzerdeutsch German speaking, with a large French-speaking minority in the west, a smaller Italian minority in Ticino and Graubunden cantons, and of course the Rhaetian speakers of Romansh in Graubunden.
Poland has several German-speaking enclaves in the territories taken over from Germany after World War II, and in addition a couple of smaller West Slavic languages (Kashubian comes to mind) that are being assimilated into Polish.
Latvia and Estonia split between them the few surviving speakers of Livonian, who live approximately on the border between the two countries.
Look at a map of Russia before the Soviet Union split up for ethnic minorities; there are quite literally dozens.
Bulgaria has speakers of Turkish and a Turkic language called Gagauz.
Austria has a few small enclaves of Ladin speakers (distinguish this from Ladino, the Spanish equivalent of Yiddish; it’s a Rhaetian dialect akin to Romansh. I believe there are also a few areas on each side of the Austrian/Slovenian border where the language of one side is spoken across the border on the other.
Italy has a wide number of dialects of Italian, and in addition the following: (a) Sard, the quite distinct language of Sardinia; (b) Catalan, spoken in a couple of cities on Sardinia; © Valdostano, which is effectively a dialect of Savoyard spoken in the Valle d’Aosta; (d) Friulian, yet another Rhaetian dialect, spoken in Friuli-Venezia Giulia; (e) Albanian, spoken along the ‘heel of the boot’ in several enclaves. There may be more, but those are the ones I’m certain of.
Romania has large enclaves of German and Magyar speakers.
Sami, the language of the Lapps, is spoken in a wide band occupying parts of northern Norway, Sweden, Finland, and Russia (Kola Peninsula).
About 10% of Finnish nationals speak Swedish, largely bilingually with Suomi. Karjala or Karelian, considered by some a separate language from Suomi Finnish, is spoken in a fairly wide belt spanning the Finnish-Russian border.
That’s a not-particularly complete list to add to what’s already been mentioned.
)