In a buffer that contains bidirectional text, setting
bidi-paragraph-direction to 'left-to-right aligns the RTL text with the
left margin like the LTR text; see the first sexp below and the top
buffer in the attached screenshot.

Another alternative to get this alignment is to leave
bidi-paragraph-direction at its default value of nil and use a space
display specification with a suitable value for the :align-to property,
but this requires a calculation that seems to depend at least on the
font used and the values of `window-width' and `string-width' for the
string used (or the corresponding pixel-width values); see the second
sexp below and the middle buffer in the attached screenshot.

It's also possible to avoid this calculation and use an :align-to value
of 0 in combination with setting bidi-paragraph-direction to
'left-to-right; see the third sexp below and the bottom buffer in the
attached screenshot.  However, as the screenshot shows, this last
alternative breaks Arabic text shaping, at least for the fonts I tried
(Noto Naskh Arabic, DejaVu Sans and Amiri).  If this is not an Emacs bug
but a limitation of the current display engine or a HarfBuzz bug,
perhaps it should be noted in etc/PROBLEMS.

(let* ((a "السّلام عليكم")
       (b "Hello")
       (buf (get-buffer-create "Test1")))
  (with-current-buffer buf
    (erase-buffer)
    (setq bidi-paragraph-direction 'left-to-right)
    (insert a "\n\n" b)
    (switch-to-buffer buf)))

(let* ((a "السّلام عليكم")
       (wa (+ (- (window-width) (string-width a)) 0.75))
       (b "Hello")
       (buf (get-buffer-create "Test2")))
  (with-current-buffer buf
    (erase-buffer)
    (insert (propertize " " 'display `(space :align-to ,wa)) a "\n\n" b)
    (switch-to-buffer buf)))

(let* ((a "السّلام عليكم")
       (b "Hello")
       (buf (get-buffer-create "Test3")))
  (with-current-buffer buf
    (erase-buffer)
    (setq bidi-paragraph-direction 'left-to-right)
    (insert (propertize " " 'display `(space :align-to 0)) a "\n\n" b)
    (switch-to-buffer buf)))