Joomla SEF URL using Language Transliterate
With Joomla 3.x
Edit below file
Root/libraries/joomla/language/transliterate.php (latin_transliterate.php - joomla 1.5; 2.5)
replace UTF8_LOWER_ACCENTS array to

Current K2 v2.6.7 is not full support Sef url replacements to some languagues (ex: vietnamese) ! K2 v.2.6.3 full support SEF URL

admin suggest using other ext, I do not like at all, actually if using other ext I will return using Joomla Content not using K2.