plugins/devkit/testData/codeInsight/languageAttribute.xml

<idea-plugin>
  <id>com.intellij.myPlugin</id>

  <extensionPoints>
    <extensionPoint name="myLanguageEP" beanClass="MyLanguageAttributeEPBean"/>
  </extensionPoints>

  <extensions defaultExtensionNs="com.intellij.myPlugin">

    <myLanguageEP language="MyLanguageID"/>

    <myLanguageEP language="<error descr="Cannot resolve language with id ''INVALID_VALUE''">INVALID_VALUE</error>"/>

  </extensions>

</idea-plugin>