Set language per post

I have a bi-lingual blog with English as the main language and Arabic. Currently I use the post “Custom Field” to indicate that the post is Arabic and I use that in my child theme to change the direction to RTL and the text translation to Arabic, however, I do that manually which means I fully customize the theme with strings in both languages and pick the correct one based on the custom field value and also set the direction for the html tags for the post title, body and comments. It is working, however, changing the theme later means I need to redo everything. I am also trying to use a comment plugin “wpDiscuz” but there is no easy way to control the language other than editing the plugin itself which I don’t want to do since I want to be able to update it easily and not redo the changes after every update.