Currently SMA-X Portal does not provide (*) tools to add new localizations for languages that aren't currently supported.
Service Manager has a toolkit to add translations, so I'd expect a similar functionality to be made available for SMA-X Portal. Even a previous version of service portal, Service Request Catalog, supported this.
In the organization i present, lack of localization possibilities strongly speaks against using a new containerized version of Portal. In fact, it blocks the adoption. Therefore I see this a major issue. Simply speaking, end users won't understand why to use a product which does not "speak" the language they do.
*) Based on answer in this thread: https://community.softwaregrp.com/t5/SMA-X-Discussions-and-Q-A/How-to-add-translations-to-other-languages-for-SMA-Portal/td-p/1627995