If you have an application foo and a database object foo\data\bar\Bar with a property baz that can be set via a form field, the name of the corresponding language item has to be foo.bar.baz.
If you want to add an additional description below the field, use the language item foo.bar.baz.description.
If an error of type {error type} for the previously mentioned form field occurs during validation, you have to use the language item foo.bar.baz.error.{error type} for the language item describing the error.
Exception to this rule:
There are several general error messages like wcf.global.form.error.empty that have to be used for general errors like an empty field that may not be empty to avoid duplication of the same error message text over and over again in different language items.
If the language item for an action is foo.bar.action, the language item for the confirmation message has to be foo.bar.action.confirmMessage instead of foo.bar.action.sure which is still used by some older language items.