some constants are not translated on the bilingual site

[jjk]

I don't understand i ?? in some fields. What is the meaning of those ??
What is the difference between ** and ??

You see those if you look at your site with Joomla language debug enabled.

**Joomla CMS**
   (text surrounded by asterisks) indicates that a match has been found in the language definition file and the string has been translated.

??Joomla CMS??
   (text surrounded by pairs of question marks) indicates that the string is translatable but no match was found in the language definition file.

Joomla CMS
   (text with no surrounding characters) indicates that the string is not translatable.

And to see any possible problems with Joomla language debug enabled, you need to click the full width gray drop-down tabs at the bottom of the page under (**Joomla! Debug Console**.