Answer
To make an additional language actually selectable at print time for a custom report, the Report Title itself specifically needs a translation registered for that language — translating other report text alone isn't sufficient to make a language appear as selectable. Critically, that translation needs to be entered in the Cust layer, not the Core layer, for a custom report — entering it in the Core layer, a mistake that's easy to make since Core is the default/expected layer for standard IFS content, is why the language wasn't showing up as selectable even after Scan Translatable Code and both cache refreshes.