RewriteEngine on
RewriteCond %{HTTP:Accept-Language} (ru) [NC]
RewriteRule .* %sitename%/ru/index.html [R,L]

RewriteEngine on

RewriteCond %{HTTP:Accept-Language} ru [NC]
RewriteRule !^ru(/.*)?$ /ru/index.html [R,L,NC]

RewriteCond %{HTTP:Accept-Language} !ru [NC]
RewriteRule ^ru(/.*)?$ /index.html [R,L,NC]

RewriteEngine on
RewriteCond %{HTTP:Accept-Language} (ru) [NC]
RewriteRule ^(?!ru/) %sitename%/ru/index.html [R,L]

RewriteCond %{HTTP:Accept-Language} !(ru) [NC]
RewriteRule ^ru/ %sitename%/index.html [R,L]