[Resolved] Internal Server Error when language parameter is added to htaccess

Background of the issue:
I am trying to resolve an ongoing issue where the language parameter is being added to the .htaccess file, making the website unusable. This issue occurs randomly on many sites we own and manage with WPML. We need to quickly replace the .htaccess file with the default WordPress configuration to solve this temporarily. As a workaround, we make the .htaccess file read-only, but this is not ideal. This issue has recently occurred on at least three sites: stellina.lv, thermex.lv, and beebikauplus.ee. A page where the issue can be seen is hidden link. The expected .htaccess configuration is: # BEGIN WordPress

RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]

# END WordPress

Symptoms:
The .htaccess file is being altered to include a language parameter, such as /en/, /ru/, or /lt/, causing the site to become unusable. The altered .htaccess configuration is: # BEGIN WordPress

RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /en/
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]

# END WordPress

Questions:
Why is the language parameter being added to the .htaccess file?
How can we prevent the .htaccess file from being altered?