These European Plates are not illegal, but they are NOT legal replacements for your US plate.
They are used as novelty or in addition to your US DMV issued plate. Not every state has a front license plate law however, so putting them on the front of your car would be ok.
Most often you could risk a fix it ticket if you had no plate on the front of your car if in a state that requires it, or if you had our plate.
States that DO NOT require a front license plate as of Jan 2019
Alabama
Georgia
Michigan
North Carolina
Arizona
Indiana
Mississippi
South Carolina
Arkansas
Kansas
Oklahoma
Tennessee
Deleware
Kentucky
Pennsylvania
West Virginia
Florida
Louisiana
New Mexico