See attached. The error messages for the French form are translated (string translation and in the classic editor). On the French front end, error messages still show up in English.
Thanks for contacting WPML forums support. I'll be glad to help you today.
1) First of all, sorry for the late reply due to a higher workload. The "Please fill out this field" string is a JavaScript string, which appears according to the browser language, not the site's language.
Also, the "Success" string works according to the browser language, not the site's language.
In order to test that, I visited the French page using an Arabic browser. (Kindly check the attached screenshot)
2) It's unrelated to the issue, but according to your Debug.info, the WP memory limit needs to be increased. PHP memory is fine, but WordPress uses 40Mb as default. The minimum requirements for WPML are 128 MB. (Kindly check this page https://wpml.org/home/minimum-requirements/)
You can increase it by adding the following code in your wp-config.php file right before the /* That's all, stop editing! Happy publishing. */ line: