I’ve never had to do this for any of my sites or client sites, but just by googling a little bit I found this:
https://wordpress.stackexchange.com/questions/133882/how-to-change-the-language-for-the-front-end-only
Check the first answer. This code that you paste on the functions.php file should, in theory, do the trick.
Before doing it though, you need to go to the backend, go to options, switch the language to the one you want on the frontend only and click Save. This should download it and make it available for use by the snippet of code. Then switch it back to, say, English.
Remember to switch ‘ar’ in this line: return ‘ar’; with the ISO languange code for the frontend languange you downloaded. Full list of ISO codes here: https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes. If you don’t find it let me know.
Hope this helps.