In assets/scss/rtl/_spacing.scss:

.mr#{$infix}-auto,
.mx#{$infix}-auto {
    margin-right: auto !important;
}

mr-auto should set an auto left margin on RTL languages.

and

.ml#{$infix}-auto,
.mx#{$infix}-auto {
    margin-left: auto !important;
}

ml-auto should set an auto right margin on RTL languages.

In the example site you can that it affects the navbar when switching to an RTL language:

The nav items should float to the left but the navbar still has an auto margin-left, so it stays in place.