Complex sentence: Japan has been known as a "haven for stolen artifacts", it wasn't until 1999 when Japanese Ambassador Koichiro Matsuura was appointed director-general of UNESCO that Japan thought about signing the UNESCO convention agreement to return stolen artifacts.
Split into simpler sentences: Japan has been known as a "haven for stolen artifacts". Not until 1999, when Japanese Ambassador Koichiro Matsuura was appointed director-general of UNESCO, did Japan begin to give serious thought to signing the UNESCO convention agreement to return stolen artifacts.