When installing in an RTL language, it should be RTL from profile selection onwards

Likely not a very hard issue to solve, assuming the installer has RTL styling in the theme already. Marking as medium.

Looks like the lang="en" and dir="ltr" on the page even if you pick Arabic. We should fix that first and foremost. If that is fixed (I just hand-edited locally), the page would look like this:

While that does not fix it completely (the indicators are on the wrong side of the step list), this page is being redesigned at #1337554: Develop and use separate branding for the installer.

All right, the problem is until the database is set up, we don't have a persistent storage to set language_default and we just don't temporarily set it to inform the theme system to act accordingly. If we do this right after configuration is inited, we get the behaviour as screenshot in #2. There is still a CSS assignment issue since later the Druplicon and the steps move to the left side proper:

So this does not yet happen as early as the profile selection step but it should. CSS at the configure page includ:

    <style media="all">
@import url("http://d8mi.localhost:8082/core/misc/normalize/normalize.css?mlsyd5");
</style>
<style media="all">
@import url("http://d8mi.localhost:8082/core/modules/user/user.css?mlsyd5");
@import url("http://d8mi.localhost:8082/core/modules/user/user-rtl.css?mlsyd5");
</style>
<style media="all">
@import url("http://d8mi.localhost:8082/core/modules/system/system.base.css?mlsyd5");
@import url("http://d8mi.localhost:8082/core/modules/system/system.admin.css?mlsyd5");
@import url("http://d8mi.localhost:8082/core/modules/system/system.theme.css?mlsyd5");
@import url("http://d8mi.localhost:8082/core/modules/system/system.maintenance.css?mlsyd5");
@import url("http://d8mi.localhost:8082/core/modules/system/system.base-rtl.css?mlsyd5");
@import url("http://d8mi.localhost:8082/core/modules/system/system.admin-rtl.css?mlsyd5");
@import url("http://d8mi.localhost:8082/core/modules/system/system.theme-rtl.css?mlsyd5");
</style>
<style media="screen">
@import url("http://d8mi.localhost:8082/core/themes/seven/style.css?mlsyd5");
@import url("http://d8mi.localhost:8082/core/themes/seven/style-rtl.css?mlsyd5");
</style>

The swapped installer sidebar layout comes via seven's style-rtl.css. Still needs to be figured out why RTL CSS is not added to the page even though the installer now recognizes it is Arabic and RTL.

Found the CSS problem too. The maintenance theme assembles CSS but the RTL CSS addition is in a hook with language module. So obviously before you pick the first profile, there are no modules enabled whatsoever. However, we should still add corresponding RTL CSS when available, so we should do a little manual intervention there and call out to the appropriate code in language.module. Thankfully drupal_get_css() already supports injecting a modified list of CSS from the outside (otherwise falling back on drupal_add_css()) so we can easily do that. Also, language module's alter is multi-run-safe, so if the module is indeed enabled, it will not re-add the CSS multiple times (if maintenance page is run on an installed site that is).

So this looks to me the complete solution. Reviews now welcome :)

Note every placement looks correct, arrow point from/to the right direction, etc :)

Quick simplytest.me testing link: http://simplytest.me/project/drupal/8.x?patch[]=http://drupal.org/files/installer-rtl-4.patch

Better title since the installer does become RTL at some point, but pretty late.

Thanks for testing!

I think the Druplicon aligning to the right is fine for now, I don't want to mix in CSS fixes here especially that #1337554: Develop and use separate branding for the installer proposes redoing the CSS altogether (and has its own RTL testing there :).

As for the simplytest.me error, I've found that at #1974912: Problems installing Drupal 8, and it does work for me with reloading this page and going forward, so I'm just doing it for now in my testing. Looks like it might be fixed in core soon given there is a working patch :)

Apart from that any other issues?

language_default is not anywhere close to being anywhere on the Request object, its a site-global setting, which you cannot really have without even selecting a profile/set up DB obviously.

Superb, thanks! I have not checked if this applies to Drupal 7. It might or might not. The span/effect of it on Drupal 7 would be much less, since you pick language after you picked a profile and then the screens showing before Drupal is properly installed are DB setup and the module batch screen. Those may show in LTR. In Drupal 8, we moved language ahead, and strive for a fully flawless multilingual experience from the first step. If someone finds this not working in Drupal 7, please reopen for backport.

This regressed unfortunately: #2240007: Regression: early installer is not in RTL after selecting RTL language :(