CSB5 Demo 
Page Properties: The first page. In Russian it sounds as Ïåðâàÿ ñòðàíèöà
NEW FEATURE: Also insert the meta tag <META CONTENT="text/html; charset=windows-1251"> into the page properties.
The content in Cyrillic letters above and below comes from FRANK.
He added it to the wish list and CSB 5 now resolves his issues!
If you point with the mouse to any link in the sequence you see a floating out (may be it is more right to say popping up) name (title) of the place the link goes.
But you see only ENGLISH PART of the title. Russian letters are not seen either in charset=iso-8859-1 , or in charset=windows-1251.
Instead of "The first page in Russian sounds as "Ïåðâàÿ ñòðàíèöà" you see only "The first page in Russian sounds as" and nothing else.
RESOLUTION: The problem reported with CSB and Trellix by Frank was due to the use of "" qoutes in the Title field in page and image properties being transferred to the title field that makes up the floating tooltip. DO NOT USE QOUTES IN FIELDS...IT BREAKS THE CODE!
It concerns the titles of the pages and images. Both languages are readable. WHEN YOU DO NOT USE QUOTES IN THE IMAGE DESCRIPTION.
But not always. The name I gave the next picture is: In Russian it means "îáðàçåö èçîáðàæåíèÿ". But you see only the English letters. Cyrillic ones are not seen.
As for big letters: - is not seen in the .tlx variant,
Russian Ñ and Ð when copied pastes as Russian - . One more example: Russian words -îáàêà (a dog) and -ó÷êà (a pen) paste as: Russian words -îáàêà (a dog) and -ó÷êà (a pen).You may try youself - the .TLX file is HERE.
Note that when you COPY PASTE Cyrillic Letters you sometimes lose them or get errors. That has not changed, It might be more related to Windows Clipboard and default keyboard settings than CSB.