[Zope] Help STX in UTF-8

I registered the help directory using context.registerHelp(directory='help') in my product's __init__.py. Then I placed UTF-8 encoded STX file in my language there, but the help via ZMI is displayed with garbled UTF-8 chars. It's unreadable, but it is not the matter of browser's Display encoding switch. It's Zope 2.7.5. What can I do to display the file correctly? -- \//\/\ (Sometimes credited as 1494 F8DD 6379 4CD7 E7E3 1FC9 D750 4243 1F05 9424.)