This mostly happens if your language has some "non-standard" characters or perhaps the whole set of characters of your language is "non-Latin".
To fix this problem, first thing to try is to change the font used on your ticket template to one of the unicode ones: all Arial fonts, all DejaVu Sans fonts, FreeMono, FreeSans, STSong Light. To change the font, you should edit the ticket template and then select the font from the Fonts dropdown menu. Changing font this way should help immediately and the problem should be fixed. However, we've witnessed several times that none of these fonts did the trick. In such cases, you would need to use our Custom Ticket Template Fonts add-on and upload a custom font which has character set specific to your language. Thorough instruction on using Custom Ticket Template Fonts add-on can be found here.