I suggest you rather add the classes to bootstrap-rtl.less where we already have some stuff for frontend.
Then run generatecss.php

This will not make necessary to add a rtl.css to Protostar and load it.

Also Careful: the "initial" value may not work with IE.

I don't understand why require to add to bootstrap-rtl if this is specific issue for protostar.
Anyway, I changed the left initial to left auto.

@oc666
Until today, even when I suggested it when Protostar was created, nobody wanted to create a specific rtl.less and subsequently a rtl.css for protostar. Thus the reason of my post.

If PLT agrees to add now a rtl.css to protostar, I have no issue with it, but should also be created a rtl.less and generatecss.php should be modify to include this new setting, i.e. add

JPATH_SITE . '/templates/protostar/less/template_rtl.less' => JPATH_SITE . '/templates/protostar/css/template_rtl.css',

line 55 of https://github.com/joomla/joomla-cms/blob/staging/build/generatecss.php#L55

the new code shoudl be added in the new .less file and generatecss.php used.

IMO the right way would be to add a class for the html or body tags when a RTL language is active. Then use it to define specific behaviors for rtl languages in a new rtl.less inside protostar. Then you only have to import it in template.less without needing to modify template index.php or generatecss.php.... And you save to have 2 css files loaded (2 requests).

@oc666 can you look at the suggestion from @phproberto so that we can get this important fix made in the core

See #7200 implementing @phproberto solution.

Closed in favour of #7200