I'm looking for a way to set the default language for visitors comming to a site built in EPiServer for the first time. Not just administrators/editors in the backend, people comming to the public site.
Depends on your setup.
If the site languages is to change under different domains you can do this. Add to configuration -> configSections nodes in web.config:
<sectionGroup name="episerver">
<section name="domainLanguageMappings" allowDefinition="MachineToApplication" allowLocation="false" type="EPiServer.Util.DomainLanguageConfigurationHandler,EPiServer" />
..and add this to episerver node in web.config:
<domainLanguageMappings>
<map domain="site.com" language="EN" />
<map domain="site.se" language="SV" />
</domainLanguageMappings>
Otherwhise you can do something like this. Add to appSettings in web.config:
<add name="EPsDefaultLanguageBranch" key="EN"/>