If you want to replace a page with another, it is just a simple RewriteRule
RewriteRule ^zh/about.html$ /en/about.html [R,L]
If you want to redirect all pages to the English counterpart, you must first capture the part after the language, and append it to en. But you must be careful and check, if it is already en. Otherwise, you would have a rewrite loop
RewriteCond %{REQUEST_URI} !^/en/
RewriteRule ^../(.*)$ /en/$1 [R,L]
When everything works as it should, you may replace R with R=301 (permanent redirect). Never test with R=301.
To have any other length prefix, use .+? instead of ... This looks for one or more instead of just two characters
RewriteRule ^.+?/(.*)$ /en/$1 [R,L]