How To Enable Debug Mode in Joomla

Any issue you find in a Joomla site, it's hard to fix because there're only few details or error messages which can help you.

A precious instrument to fix it is enabling "Joomla debug mode" in Global Configuration, which allows you to see many details about your site, including:

Database queries
Profile Information
Session Data
Memory Usage

To enable Joomla debug mode:
Via backend (Joomla's Administrative Interface), access the page Site => Global Configuration

Amongst various other parameters, you'll find also "Debug Settings" Group.

Setting it to “Yes”, Joomla will add debug information on every page, below the normal web page area. This information will include various forms of diagnostic information, including the full database queries used in generating the page.
The default setting is “No”.
The mix and format of diagnostic information presented by the debug function may be altered by editing the parameters of the Debug - System plug-in (Extensions > Plug-in Manager).

Important Security Note:
the debug information displayed with this parameter set to “Yes” is visible to all users and thus may be a security risk if used on a public web site, but the debug information is visible even if Site Offline is set to “Yes”.
So, it's higly advisable to put site offline previous of enabling debug mode.

Debug Language.
If set to “Yes”, Joomla will activate features designed to help debug language translations for a Joomla site. This includes the addition of language translation information to page debug information (requires Debug System also set to “Yes”), and markers to show translated content within the web pages themselves.