After logging into Wikidata, go to https://www.wikidata.org/wiki/Special:NewItem?uselang=en and from the language dropdown, select "ar" for Arabic. The placeholders for the textboxes in the form will look like below:
{F31844695,size=full}
If this is meant to be a text in English, then its directionality shouldn't be changed to right-to-left.
Conversely, if the reason we are showing the local name of the language (ie. "العربية") is because we want the user to is about to enter a text in the selected language (and, after all, Arabic is an RTL language) then the entire placeholder should be replaced with an Arabic phrase that reads well in RTL (e.g. "أدخل التعريف بـالعربية" for the Label box, etc.)
In either case, having a placeholder in two languages is cute but useless. All of the placeholder string should be in one language.