Utf-8 Character Encoding Problem
HTML files are typically served as text/html. The character encoding (or ‘charset’) of this file is UTF-8. If the page is not served as utf-8, there occurs character encoding problem.
If the page is cached, the page may not be called as utf-8.
Solution
WPFC adds charset rule automatically but in some servers <FilesMatch> does not work so you need to try <LocationMatch> as below.
You need to add the following rule into htaccess to serve the files with the extension .html as UTF-8.
AddDefaultCharset UTF-8
Internal Server Error (500)
If you get Internal Server Error (500), you need to remove <LocationMatch> and add only the following rule.
AddDefaultCharset UTF-8