TeXShop contains a Dutch localization Maarten Sneep, a French localization by Jérôme Laurens, Hendrik Chaltin, and Jean-Claude De Soza, a German localization by Keith J. Schultz, Sascha Beverungen, Martin Kerz, Max Horn, and Lukas Christensen, an Italian localization by Giuseppe Carlino and Nicola Vitacolonna, a Japanese localization by Seiji Zenitani, Yoshihisa Okazaki, and Yusuke Terada, a Chinese Localization by Linus Yang, a Spanish localization by Juan Luis Varona, a Romanian localization by Roxana Tenea Teleman and Andrei Teleman, a Portuguese localization by Paulo T. Abreu, a Portuguese Brazilian localization by Emerson Mello, and a Korean localization by Karnes Kim. Thanks!
In Japan, modified versions of TeX and LaTeX called ptex and platex are sometimes used. TeXShop has been modified by Makoto Inoue, Seiji Zenitani, and Mitsuhiro Shishikura to deal with these versions. See http://macwiki.sourceforge.jp/cgi-bin/wiki.cgi?TeXShop for further details. Ptex and platex are available at http://www.ascii.co.jp/pb/ptex/. Dvipdfmx is available at http://project.ktug.or.kr/dvipdfmx/. A dvi previewer for Japanese is available at http://macptex.appi.keio.ac.jp/~uchiyama/macptex.html.
For more details on these modifications for Japan, see the following section.