Sometimes, you can successfully run a U.S. application with KanjiTalk and have a Kanji script appear in the data space and print in a Kanji script to a LaserWriter IINTX-J. However, if it hasn't been localized properly, you may encounter many more issues, possibly serious, compared to mixing and matching applications and operating systems localized for other countries. Most English applications don't support the 2-byte character set. Exceptions exist with applications like Excel or WordPerfect.
A U.S. application that uses the Script Manager, which makes it possible to support non-Roman scripts: Arabic, Hebrew, Japanese, and Chinese, may be functional (with minor incompatibilities) or usable (with somewhat minor inconveniences). Some likely incompatibilities include text-positioning issues requiring you to press the backspace key twice, incorrect date and time formats, different numerical and character delimiters, and so on. (Remember: Kanji uses a 2-byte-wide word.) With other less-compatible applications, serious system faults may occur at any time, including when changing font parameters or printing a PostScript font.
Generally, you may have fewer issues using graphic-oriented packages than text-oriented packages. Also, you can use any operating system on any Macintosh, keyboard differences aside.
Although you can usually use a non-U.S. (or other) word processor with the U.S. Macintosh OS, or vice-versa, we do not recommend using non-Kanji applications with the Kanji OS, or vice-versa.