Message: Since the default language isn't accessable through the API, and the Runtime method lookupDesignation requires that a language be provided, it is hard for a CTS client to know what a good language is to ask for. They can get a list of all available languages, but they don't know what the default is.
Other methods like lookupConceptCodesByDesignation specify that language is optional, and they use the default language if none is provided. This method does not specify that language is optional, so it throws an exception if you don't pass a langauge.
My two suggestions are:
a) make the language parameter of lookupDesignation optional.
b) specify that the first language returned in the array of supported languages is the default language.