1) Checkered Garter Snake. Sometimes known as Marcy's garter snake, or Marcy's checkered garter snake. T. marcianus marcianus.

2) Plains Garter Snake. Sometimes known as Great Plains Garter Snake. T. radix. One of Don Belnap's photos I believe.

3) Questionable. plains garters and checkered garters often share similar appearance, depending on location. This snake has features that appear to be a blend of the two species. However, in certain locations, pure checkered garters can look like this. So can pure radixes. This snake could be one, the other, or have both in it, which wouldn't surprise me if it was CB in Europe. Plains X checkered are well documented hybrids in captivity. If this snake is wild caught, location data would be needed to confirm it's identity. Without that, as Joe said "It's a guessing game"