In order to rename the "Priorities" text in the issue navigator, you may need to edit the following properties file in the language pack:
The following property value in the Issuenavigator_en.properties:
The language pack is a JAR file located in
<jira-install>/atlassian-jira/WEB-INF/lib. For more information about Customizing Text in JIRA, please refer to Customizing Text.