Well I'm a bit irritated to see that icons for external links on pages translated to Arabic (and properly tagged by Mediawiki itself to use a RTL direction) and not properly presented: the icon should be different (mirrored) and displayed to the left of the link, not to the right.
This is a problem of the global CSS stylesheet used on some multilingual wikis like Meta. When these translated messages are posted via MassMessage to other minolingual RTL wikis (such as Arabic Wikipedia), the default CSS of these wikis are properly setup.
I think that stylesheets used on multilingual wikis (Meta, Commons) should be tuned to be really international and fix their RTL vs. LTR direction. Mediawiki has suitable global classes ("mw-content-rtl" vs. "mw-content-ltr") for its wiki-generated content which can be used in CSS selectors (these selectors don't apply to the site-wide stylesheet that setup the navigation bars around the content in its default UI language or using the user's language, for its parts that are translated)
Note that some wiki contents are generated by templates, and many templates already use "mw-content-rtl" vs. "mw-content-ltr" classes for the layouts they generate (e.g. for tables or lists): but if the content generated by these templates create custom presentation (e.g. navbars, infoboxes, galleries of images), beware of not assuming that tables will be necessarily rendered in LTR direction (with the 1st column to the right)
Beware with custom styles adde in that content, notably the side of margins, paddings, borders, and "left"/"right" alignment. Beware also about the choice of images, notably those including prerendered text or direction-dependant iconography such as arrows. Note that not all "left" vs. "right" have to be replaced: some border styles for buttons for example will be inconsistant if this is used to create 3D effects or shadows whose orientation is independant and should remain homogeneous independantly of the text they contain.
To handle these tricky cases, there are useful templates like Tempalte:Dir which takes a language code parameter and allows to return two different values for LTR vs. RTL layouts. The default return values are "ltr" or "rtl". The default language code is the page's default language (not necessarily the default site language or the user's UI language selected by the ULS interface or in user preferences): identifying and tracking which language is used requires passing the languager codes as explicit parameters with the correct default. This takes some time to dersign that properly in templates, but the most used utility templates for navboxes, banners, and infobox are normally now already designed to support passing the expected language.
Whever a tempalte will display contents translated (and laid out) in the page language or in the user language depends on its use. Generally, banners (for informing users directly about what they should do and that are not directly part of the page content but attached as metadata) should use the UI language; other templates like infoboxes and navbars should use the page language: nowhere you can assume than one of these two languages will be the same as the default sitewide language of the wiki (which is only a default for the page language for pages not intended to be translated, but never a valid default for the UI language which can always be something else on any monolingual or multilingual wiki)
But even monolingual wikis now have some parts which are translated with the translation tool to address users directly in their language. In that case, these parts will be in the UI language and should use "autotranslation" technics, or use an explicit page language set by the page naming convention: this is the case for wiki documentation pages or community policy pages, or some "ambassy" pages.