There are already multiple mechanisms by which an app can learn about a user's language preferences: For web-based SMART applications, language preferences are available in HTTP headers and in JS (see https://www.w3.org/International/questions/qa-lang-priorities.en, https://www.geeksforgeeks.org/how-to-detect-the-browser-language-preference-using-javascript) For native SMART apps, language preferences are available in native OS APIs (e.g., see https://developer.android.com/reference/java/util/Locale) For all SMART apps, using OpenID Connect to retrieve a fhirUser can provide detailed language preferences ( http://hl7.org/fhir/patient-definitions.html#Patient.communication, http://hl7.org/fhir/practitioner-definitions.html#Practitioner.communication,  and so on)