It really would be nice if you get informed if you change a show message action template in one language, that there are templates defined for other languages as well.
For instance by displaying a warning in the error log, if the number of parameters defined don't match the number of parameters used in the template.
Or just by displaying a popup, asking or informing to update the other languages as well.
Now it is far to easy to forget about updating the template text of the other languages.
BTW You might be right. If one adds a higher placeholder in the template it is logged as an error. For instance:
Place holder index 666 is greater than 29, the number of parameter(s). [English, United Kingdom]
But not the other way around.
Come to think of it; the usage of {#} in the text is not allowed or is there some escape character if you really wan't to used that text?
I already reported this last tuesday, but got the response it sounded like a feature request. Hence this post. This should be known as request (55100) @ Mendix.
This seems like a bug to me, not a feature request.