As I learned from FreeBSD, Linux etc., there is mostly a chaos about multilingual apps. If the app is based on GTK+, it has the possibility of different languages, QT too, but, there is a lot outside only in English with TK/TCL or xview, every app has its own way how to be multilingual.
As BeOS from scratch supports UTF-8 encoding, it would be a good idea to support i18n around the whole GUI, which means, if I’m Polish, I can switch to Polish language and all apps are Polish too, like it is realized with the Open Tracker, but goes a lot broader. That means the input method too, so, if Haiku supports multiple users, every user can use his own language and write, maybe, in Japanese Input Method at a Hungarian interface. Only one click away. He/she can switch the keyboard layout the same way, I like a German interface but have often to write in Chinese traditional characters, but I like to use my German keyboard for there are a lot of symbols hard to find at English keyboard layout with German letters printed on.
If Haiku wants to become successful, here is a very important feature that other OS do not support. For instance, if I want to use Windows in Chinese and German, I have to install two different MS Windows, If I want to have using different languages users at Linux, I can give KDE or GNOME the order to be Chinese, Japanese etc. But, a lot of apps will not be translated into the target language.
The only way to make this possible would be a "Language Kit" what all apps can use.
I saw a very good idea at QNX with Chinese: because Chinese owns a lot of characters, the fonts will be immense big in space, TTF or Type1 will have 12, 16 or more MB. So, QNX uses special stroke fonts at the desktop, very clear, they do not use much space, so they are rendered well and fast. Only at printing time you can choose a TTF or PS font to have a nice output. (Here’s an explanation http://www.qnx.com/developers/docs/momentics621_docs/cpim_en/user_guide/about.html or http://www.qnx.com/developers/docs/6.3.0/photon/prog_guide/fonts.html – here the information from Bitstream http://www.bitstream.com/font_rendering/products/dev_fonts/sb_cjk.html) – together with the “Language Kit” it would be a big step to the future for Haiku.