self-answer:
We choose to import tables sql content from phpmyadmin (or manager backup function), and turned in the sql files for each create table declaration :
ENGINE=MyISAM DEFAULT CHARSET=latin1 COMMENT='xxxxx';
into
ENGINE=MyISAM DEFAULT CHARSET=utf8 COMMENT='xxxxxx';
(be sure to use utf8 text editor - pspad for instance )
them reimport using phpmyadmin
I first try this with the modx_content table only to check potential side effects, and processed the whole database when we where sure it was ok.
Now the Cyrillic characters are stored and rendered correctly.
Still issues with transliterateAlias, see other post.
Thank you
[edit]
Without TranslierateAlias, it’s simple to modify /www/manager/processors/save_content.processor.php after line 674 where the accented characters are converted and adding an iso9 cyrillic transliteration table in the $replace_array array
'А'=>'A','Б'=>'B','В'=>'V','Г'=>'G','Д'=>'D',
'Е'=>'E','Ё'=>'YO','Ж'=>'ZH',
'З'=>'Z','И'=>'I','Й'=>'J','К'=>'K','Л'=>'L',
'М'=>'M','Н'=>'N','О'=>'O','П'=>'P','Р'=>'R',
'С'=>'S','Т'=>'T','У'=>'U','Ф'=>'F','Х'=>'X',
'Ц'=>'C','Ч'=>'CH','Ш'=>'SH','Щ'=>'SHH','Ъ'=>"'",
'Ы'=>'Y','Ь'=>'','Э'=>'E','Ю'=>'YU','Я'=>'YA',
'а'=>'a','б'=>'b','в'=>'v','г'=>'g','д'=>'d',
'е'=>'e','ё'=>'yo','ж'=>'zh',
'з'=>'z','и'=>'i','й'=>'j','к'=>'k','л'=>'l',
'м'=>'m','н'=>'n','о'=>'o','п'=>'p','р'=>'r',
'с'=>'s','т'=>'t','у'=>'u','ф'=>'f','х'=>'x',
'ц'=>'c','ч'=>'ch','ш'=>'sh','щ'=>'shh','ъ'=>'',
'ы'=>'y','ь'=>'','э'=>'e','ю'=>'yu','я'=>'ya',