Steps to reproduce the issue

1. Under System > Global configuration set "debug language" = yes.
2. Set "debug system" = yes.
3. Save & close
4. Under Extensions > plugins open the "system debug" plugin and set "Log Deprecated API" = yes.
5. Save and scroll down and expand "Untranslated Strings".

Expected result

On the bottom of the page there should be a debug section "untranslated strings" and in this section there should be only the item "None".

Actual result

The debug section "untranslated strings" is showing:
S_MESSAGES_LOGGED="messages logged**"

System information (as much as possible)

Source code from stage cloned. Currently, this 3.6.0-alpha2-dev.
PHP 7.0.6

Additional comments

The problem lies in the nested language string call in JROOT\plugins\system\debug\debug.php

In line 1785, there is something like:
... JText::sprintf(JText::_('PLG_DEBUG_LOGS_LOGGED'), $logEntriesTotal) ...

This is wrong. It must be either:
... JText::sprintf('PLG_DEBUG_LOGS_LOGGED', $logEntriesTotal) ...

or:
... sprintf(JText::_('PLG_DEBUG_LOGS_LOGGED'), $logEntriesTotal) ...

I don't know which solution is the recommended.