3.4.5. Messages
Text in the templates is loaded from message bundles. A theme that extends another theme will inherit all messages from the parent’s message bundle and you can override individual messages by adding <THEME TYPE>/messages/messages_en.properties to your theme.
For example to replace Username on the login form with Your Username for the mytheme create the file themes/mytheme/login/messages/messages_en.properties with the following content:
usernameOrEmail=Your Username
usernameOrEmail=Your Username
Within a message values like {0} and {1} are replaced with arguments when the message is used. For example {0} in Log in to {0} is replaced with the name of the realm.