Suggestion of some alternatives based on en.example.com/aout
Using the command-line interface
Starting apache with
sudo bash -c 'export _LANG=en-US && /Applications/MAMP/bin/startApache.sh'
and in .htaccess
DefaultLanguage ${_LANG}
from previous answer by anubhava
Using FilesMatch in .htaccess
<FilesMatch "^en\.">
DefaultLanguage en-US
</FilesMatch>
<FilesMatch "^en-US\.">
DefaultLanguage en-US
</FilesMatch>
<FilesMatch "^es\.">
DefaultLanguage es
</FilesMatch>
from: http://www.askapache.com/htaccess/using-http-headers-with-htaccess.html#FilesMatch_Directive
Using mod_rewrite in .htaccess
RewriteEngine On
RewriteBase /
#get language from host name
RewriteRule ^ - [E=LANG:en]
RewriteCond %{HTTP_HOST} ^(es|fr|en|se)\. [OR]
RewriteRule ^ - [E=LANG:%1]
Header always set Content-Language "%{LANG}e"