Fake mirroring
Most bilingual products flip direction and stop. The layout reverses, but filter order, progress steps, and search behaviour still follow the English mental model. The result looks Arabic and behaves English, which is more disorienting than an untranslated interface.
The tell is usually a wizard: the visual order reverses but step one still sits where step one sat, so the numbers count backwards.
Typography is not a setting
Arabic is a connected script. Letter-spacing, which tightens Latin display type nicely, severs the joins and makes Arabic words illegible. The same value that improves an English headline destroys the Arabic one beside it.
Arabic also renders optically smaller at an identical pixel size and needs more leading for its dots and diacritics. A single type scale serving both languages is always wrong for one of them.
Where conversion is actually lost
When Arabic pages convert below their English counterparts, the cause is rarely the copy. It is a form: labels that read as questions in one language and commands in the other, validation messages that arrive in English, and number fields that fight the input method.
One form component that swaps direction, language, and validation cleanly fixes more than any amount of retranslation.