We need to generate "template" URL for the javascript code something like /content-type/edit/12/

{languageCode}/{fromLanguageCode}

path( "ezpublish_admin_content_type_edit_translate", {'typeId': contentType.id, 'languageCode': '{languageCode}

', 'fromLanguageCode': '

{fromLanguageCode}'} )
generates
/content-type/edit/1/%7BlanguageCode%7D/%7BfromLanguageCode%7D

which is not suitable.

So we need a js_template_path() function would generate the template url.
Example:

js_template_path( "ezpublish_admin_content_type_edit_translate", {'typeId': contentType.id } )
generates
/content-type/edit/12/{languageCode}/{fromLanguageCode}

ie it replaces the given parameters like path() and for the missing parameters, the URL is generated with the "placeholder".