I created and linked [[:en:Module:Age]] and [[:th:Module:Age]] together via [Add links] button on the sidebar. The software linked these sitelinks as I expected, but it filled labels incorrectly; it didn't include namespace in labels. This is inconsistent with [[:wikidata:Help:Label]] which said "If a page normally displays on Wikipedia with a namespace in front of it, you should keep that namespace in the label."
I know that this is only a policy for Wikidata, not for all Wikibase sites. Perhaps Wikibase extension should be configurable for enabling/disabling filling namespace automatically. However, it should filling namespace by default to distinguish pages from different namespaces.