On why only have one language, maybe size, licencing for a given translation having different rules than the rest, focus in a given market, position of the planets... Could be N reasons.

On if you can trade games, only vaguely familiar with Canadian and American laws, but afaik, nothing should be stopping you? Maybe aside from a game being explicitly forbidden (dunno if either have such a list), or if they break some law.

Also more broadly, among people with Japanese as a second language, native Japanese speakers living in the two countries, collectors, resellers and import sales, I'd say you have a non-zero chance of finding someone interested.

And about age-rating boards, I don't think I've seen them have Executive/Legislative powers in either countries, only as a recommendation/guideline measure. May be good to confirm on that though.