From b2evolution manual

In order to use UTF-8 (UTF8) for your blog, basically, all that you have to do is go to Blog Settings > General and select the appropriate locale for your blog. The selected locale will set the languagen charset, country, flag and time formats for your blog.

If you don't see the appropriate locale for your country or if you would like to customize it, go the Global Settings > Regional. There you can either enable additional locales or create custom ones of your own.

For UTF-8, make sure you see "utf-8" in the Charset field of the locale you plan to use.

See also:

• My own flag and locale
• Localization