If I understand you correctly then you need to do it:

elgg_set_http_header("Content-type: text/html; charset=UTF-8");

But this in html exist already, so can you explain what do you want to do?

Why do you think UTF8 not be able to display/save those characters?

The recommendation from W3C the world standard organization is to use UTF8 https://www.w3.org/International/questions/qa-what-is-encoding

And i found a list of all supported UTF8 characters http://www.fileformat.info/info/charset/UTF-8/list.htm and it includes a lot of accents

yes you're right I handed UTF-8

but why there are messages that do not show accents or stored data do not contain accents while the original data had accents

maybe filtering by CKEditor or HTMLawed? I don't know exactly, but those are good places to start looking

ok