Multilingual word processing: mainly about Mellel for OS X
As they say in Wikipedia, this entry is a stub. I think more Mac users should know about the extremely fabulous Mellel, which permits the user to do things previously possible only in Adobe Illustrator or Indesign or Quark -- with this difference, that it was never possible to type merrily along in Japanese or Arabic or Hebrew or Korean in any of the full-fledged desktop publishing and image manipulation programs. Since proper design and/or typesetting programs cost upwards of $600, and Mellel costs $49, anyone who owns a Mac that runs OS X can have only one reason not to have Mellel: they didn't know it existed.
I recently came across Christopher Bolton's webpage, Japanese on Your Mac, which gives extremely comprehensive coverage of the use of Japanese on a Mac. I thought it might be useful to supplement this, so I am posting a guide to inputting Japanese in Mellel. (Much of the guide is also applicable to the use of Japanese in other word processing programs for OS X -- a topic for which Apple has provided no documentation in the 8 years since the launch of OS X.) I have also attached a couple of pages displaying some of Mellel's formatting capabilities in Arabic and Japanese. The thing to note is the ability to change the fill, stroke weight, stroke style and background colour of each character -- something offered by no other word processing program.
Using Japanese in Mellel for OS X (PDF)
Text Formatting in Mellel -- Japanese and Arabic (PDF)
Word for Macintosh supports vertical typing and ruby (phonetic glosses), which are not yet available in Mellel; this will clearly matter to some writers in Japanese.
Word doesn't offer the design-standard formatting options of Mellel; it has hysterics if asked to produce cursive text running from right to left (Arabic, Farsi and such). The last point makes Word useless and Mellel indispensable for those who have need of such languages (some of whom, of course, will also be using Japanese).
I give below a sample of Arabic in Mellel and in Word:
Mellel:

Word:

Mellel has written the Arabic word correctly, from right to left, using the short form of nun (like our dotted i) which is used at the beginning and in the middle of words. Word has completely scrambled the letters and printed them all separately (read right to left the word now says "D O N T I N I N").