Right now there is no option on canvas app to change the UI language for users. If user has multilingual environment they need to change ui language separately from mode driven side and from canvas app side. Function or connector to make this possible is needed.
This comes in use, if you have eg. bilingual apps and where you use the standard translation method to translate the modeldriven app and eg. the choice fields. When those choice fields are used in canvas app, the language that user sees is based on UI language set in the modeldriven side of the app. Since most likely the canvas app contains text, titles etc. that cannot be translated in standard solution translation method there should be able to changes users UI language from the Canvas app. That way the language set ups will be smooth and same within the different apps that end users are using. Or if end user uses only canvas app, they have no possibility to change the UI language.