The Customer Account Fields app allows you to customize most of the text that is shown to users. Visit the Settings tab to configure each message.
Validation Error Placeholders
When a field fails your custom validation rules, you can specify the messaging. Note that the message is global for all errors of that type.
For example, if you specify that a field must contain a certain value, like "John", the default error message is: Value must contain "John". You can reword this to be something else, like "John is missing", using the following: @value is missing.