話す

How it works

The whole clause 日本語を話すにほんごをはなす (speaking Japanese) acts like a noun here: の (nominalizer) bundles the action into a "thing", which は (topic) then makes the topic. So it literally says "the speaking-of-Japanese, as for it, is fun". を (object) still marks 日本語にほんご (Japanese) as the object inside that clause, and 楽しいたのしい (fun) describes the whole bundle.