Is it legal to put an european license plate on the front in pennsylvannia?

Answer:
No
The following Pennsylvania laws I found on this matter
C.S.A. (Consolidated
Statutes, Annotated):

§ 1372. Unauthorized transfer or use of registration.

No person shall:

1. allow a registration card or plate or permit to be used by any
person not authorized to use it or on any vehicle other than the
vehicle for which it was issued;
2. use any registration card or plate or permit unless authorized
to do so; or
3. display a registration card or plate in, on or in connection
with any vehicle other than the vehicle for which it was issued.

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no

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