Parameter deprecation messages will appear in the interface, so should be translated into the same languages as the template description. Currently, we only support a string in the default language.
See T238411: Structured localization framework for templates.
Good catch @awight , are you planning to work on this in your efforts to improve templates in the VisualEditor?
I'm not sure yet--it's helpful to know that you agree this should be a feature, but prioritizing the work is a separate question. We'll respond here once we can answer!