Brilliant! You could set up all sorts of challenges, your own personal ones, group ones, ‘rite of passage’ types, temporary ones e.g. at an eisteddfod. If the poster was kept behind the counter, then interaction in Welsh to ask to scan it is a conversation opener, and a way of the person in the location knowing that participating in the scheme is helping bring the public into their business.
But as you suggest, probably best to keep it simple at first. I like your proposed architecture, much more open than a wordpress plugin.