When publishing an app in the Google Play store, I would like to use the ® symbol for the name of a product that is not registered as a trademark in the USA, but it has been registered as such in the Benelux. Is it fine to use the ® symbol there?
Yes, it appears that you can. In § 906.01 of the Trademark Manual of Examining Procedure (TMEP) it states:
In addition to the United States, several countries recognize use of the symbol ® to designate registration. When a foreign applicant’s use of the symbol on the specimens is based on a registration in a foreign country, the use is appropriate.
Crucially, the country it is registered in must provide for the use of the ® symbol in its law. A discussion of this can be found in the Trademark Trial and Appeal Board case Brown Shoe Co. v. Robbins (PDF, pg. 14-15).
Since the TMEP lists the Benelux countries as providing for the use of the ® symbol, your use would be appropriate.