What is the best way to setup Right to Left language locale in Craft?
You can use the locale as per below examples in your _layout.twig file. 

&lt;body class="{% if craft.locale == 'ar' %}rtl{% endif %}"&gt;


Your css would then be: 

.rtl {direction:rtl}


And in your &lt;html&gt; like so:

&lt;!DOCTYPE html&gt;
&lt;html dir="{% if craft.locale == 'ar' %}rtl{% endif %} lang="
{{craft.locale}}"&gt;
&lt;head&gt;


https://www.w3.org/International/questions/qa-html-dir