Go to Administration -> Site Settings

Select your preferred language in  'The default language for your site' option

No, this is not working, still for guests(without account) is shown only English.

this can happen if the language you selected isn't translated completely.

When selecting the default language under the site settings you can see the percentage translated behind the language name

the system will always use English as the ultimate fallback, as the system is build with the English language as base

Ok, many thanks, now I understand. So other translated language must be always 100 percent?

If you don't wish to have any English translations or your site, then yes.

The Elgg core languages are maintained in Transifex and pull before every new release (they need a certain percentage complete before they are included). See https://www.transifex.com/elgg/ and http://learn.elgg.org/en/stable/contribute/i18n.html

For plugins the developers need to include custom translations.

If you wish to easily translate missing translations on your site, check out this plugin https://elgg.org/plugins/385116, with this you can also let your users help with translating if you wish.

Thanks, so when I use this plugin, so I can using different language as default language?

Sure.

Thank you very much !