Answer
This has been traced to the affected PC's Windows 'language for non-Unicode programs' setting not being set to Japanese — the IEE Navigation Menu's Japanese text is apparently rendered through a non-Unicode code path (unlike the Context Panel's Japanese text, which renders fine as Unicode). Changing that specific Windows setting to Japanese on the affected client PCs resolves the blank-box rendering issue in the Navigation Menu, without needing any IFS-side configuration change.