If you see squares in the game instead of text, then you are missing the font called Arial Unicode MS.
It's a common error in all new muonline clients so make sure to follow the easy steps listed below.
1. Close game client
2. Download the missing ARIALUNI.TTF font from http://muonline.download/files/ARIALUNI.TTF
3. Right click on the file you just downloaded and click Install
4. Alternatively you may download the missing font and copy it to C:/Windows/Fonts or Control Panel > Fonts
5. Start game client