Recently migrated to Prince on Ubuntu for converting HTML to PDF and we are extremely happy with this software.

We need your help in solving a warning related to fonts
"warning: no glyphs for character U-------, fallback to '?' "

First we installed 'msttcorefonts' fonts as explained in the documentation. Then we got this problem for Japanese and Korean languages. once we install Jap and Korean fonts the problem went away.

The problem is that we can have multiple languages in our HTML(generated from user input), how do we convert this to pdf without running into issues every time user enters a new language.

In general you can't solve this for every possible language, as the user could always use a very obscure language that you are unlikely to have fonts for. (If you look at Unicode you will see that it includes many historical and extinct languages, for example).

If you install fonts for Latin, Cyrillic, Greek, Chinese, Japanese, Korean, Arabic, Hebrew, Hindi, and other Indic scripts, then you have covered about all the scripts that Prince supports at this time.