Answer
Check whether you've translated the report's own title (the report definition's name itself, searchable in Text Translations within percent-sign markers) — not just the field labels/content that get printed inside the report. The print-language dropdown is driven by whether a translation exists for the report title specifically; translating only the printed content's labels isn't enough to make the language show up as a selectable print option. Once the report title itself has a translation and the Language Cache is refreshed again, the language appears in the dropdown.