Things are getting better these days, but up until very recently the default OSS stance towards internationalization was "Unicode is hard, let's go shopping". It is hard to get managerial approval to deploy a system which can't run on his computer without corrupting display of the characters he inputs and whose documentation his engineers can't read.
And that is far too common. On the other hand, modern desktop software (either OSS and MS) is, thankfully, quite OK, nowadays.
1. don't implement/misimplement joining/shaping behaviour
2. don't support vertical left-to-right layout
Though to be fair, for the better part of a decade MonTex was the only solution for anyone who wanted to write a document in Mongolian/Manchu/Dagur/etc.
http://babelstone.blogspot.com/2008/10/vertical-layout-in-ie...
Just three years ago, I set up Firefox on my Korean friend's computer, and found out that he couldn't properly type Korean in his webmail program, while IE worked fine. You can guess what browser he decided to use.
That particular problem is fixed now, but the simple truth is that Microsoft got into the game way ahead of everyone else.