There is an amazing set of new features and improvements in Drupal 8 core, and I'm going through them one by one so you get to know what is coming and also I can highlight the issues that you can still help with to make these better. I believe Drupal 8 will be a huge release for multilingual, but it can always be even better of course.
RTL drupal 8 main menu
when i change language to arabic, only main menu still in LTR, please tell me how can i change it into RTL ?
could be due to your theme
Did you try with the built-in Bartik theme? Unless it is broken there as well, it is likely due to your site theme.