If unspecified with the request, the product (upon determining the country for the Verification) identifies all the languages spoken in the region and picks the most appropriate language available.
You would need to specify a locale with your request. You can find more information about the &lg parameter here.
If the primary language is not listed in the above documentation, then the SMS and TTS call will default to using "en-us" as the language for the TTS call, and US-english for the SMS.
Please note that for the US and Canada, the SMS is sent over short code so will always be in English.