how to load RTL style in RTL language
Drupal 8 uses libraries, and you can load these in templates. One way of achieving language direction dependant libraries would be to use the html_attributes['dir'] variable in html.html.twig and load different libraries depending on the value:

{% if html_attributes['dir'] == 'ltr' %}
  {{ attach_library('your_theme_name/styles') }}
{% elseif html_attributes['dir'] == 'rtl' %}
  {{ attach_library('your_theme_name/styles_rtl') }}
{%- endif -%}


Set up a couple of libraries:

styles:
  version: 8.x-1.0
  css:
    theme:
      css/styles.css: {}

styles_rtl:
  version: 8.x-1.0
  css:
    theme:
      css/styles-rtl.css: {}