The new Instant App Language Switcher in ArcGIS Online is a great feature. After selecting a language, the "translation configuration" dialog box lets you fine-tune the wording to better match your needs.
However, many smaller languages, such as many Native American languages, are not available as selectable options. To support these cases, users should be able to configure a custom translation option in the "translation configuration" dialog box. This would allow them to define their own language label and translations, making the app more inclusive for communities whose languages are not currently listed.