The yen sank Monday to its lowest level against the U.S. dollar in over 23 years on expectations of a further widening of the interest rate gap between Japan and the United States. The Japanese currency briefly fell to around 135.22, its lowest level since October 1998, in Tokyo trading on speculation the U.S. Federal Reserve will continue its aggressive monetary tightening following the release of higher-than-expected inflation data for May. At 1 p.m., the dollar fetched 134.91-92 yen compared with 134.35-45 yen in New York and 133.59-62 yen in Tokyo at 5 p.m. Friday. The euro was quoted at $…