This Package action will add a new translationkey to the umbraco language file of the selected language. The undo option makes sure that the key can be removed again at uninstall.
Alias: AddLanguageFileKey
<Action runat="install"
undo="true/false"
alias="AddLanguageFileKey"
language="en/da/de/es/fr/it/nl/no/se/sv"
position="beginning/end"
area="template"
key="demo"
value="This is a demo string" />