This text is contained in searchbnf.conf. There is a searchbnf.conf in $SPLUNK_HOME/etc/system/default/. You should not modify it. If your application has its own custom python search commands, your application can include its own searchbnf.conf to describe the commands to the search assistant.
Sorry for the delayed follow-up, I was away for a few days. The strings you are looking for are contained in the messages.pot files in $SPLUNK_HOME/lib/python2.7/site-packages/splunk/appserver/mrsparkle/locale.
HOWEVER: it is not recommended that you modify this file. It will be overwritten whenever you upgrade, and modifying default Splunk files is not a good practice as a general rule. It can make it difficult for customer support to help you and can have unanticipated effects in your installation.