Ryuch

Works on Wikipedia App

Type contents of sample Wikipage in the langauge using the ime of 4 paragraphs and 200 words. Include common punctuation marks, different heading levels etc.
Include symbols, special characters in the language, styling elements in the content including bold, italic, heading levels etc
Use keyboard as well as icons for doing style changes.
Use delete, backspace, navigation keys(left, right, up, down, page up, page down, home, end) while typing. (Home/end are definitely different on Mac.)
1. While using delete / backspace, the character must change as they do elsewhere.
2. Navigation keys must behave like they do usually while navigating between text. Control+<left/right> should navigate between words properly (word endings). (control-left/right is different on Mac.)
Try escaping special characters for which transliteration rules exist and use them in the text.
Turn off the IME and input some text in latin. Turn on IME again and input text. Look for formatting issues. (+1 - thanks for remembering that. Latin is probably the most common case. Switching to other layouts in general may be relevant, too.)
Copy paste bulk text from existing language wiki pages and try editing using the ime.

아리랑랑 랑 아랄ㅣㅇㅛ. 아리랑 로 넘ㅇㅓ간다. ㅡㄹ ㅗ ㄱㅏ신ㅡㄴ ㄴㅣㅁ은 도 서\ㅇ난다.

Mac OSX

한글 시험합니다.

Ubuntu

글 험 다.

Windows XP

한글 시험합니다.