The question you left without asking
A survey of 2,715 adult immigrants with limited English proficiency at eleven community health centres across the United States compared visits where a clinic interpreter was used against visits where the clinician spoke the patient's own language. The people who worked through an interpreter were more likely to report having a question about their care that they wanted to ask and did not: 30.1 percent against 20.9 percent.[1] That is the gap this page is about. Not a misunderstanding, not a mistranslation, simply a question that stayed inside.
The figure deserves to be read with its limits attached. That study looked at Chinese and Vietnamese immigrants in the United States rather than Arabic speakers, it was in general medical care rather than dentistry, and the comparison was between an interpreted visit and a language-concordant one rather than between a first and a second language in the strict sense. The same study found no meaningful difference on three other communication measures, and no difference in how many people rated the care they received as excellent or very good.[1] What survives all of those qualifications is the one finding that matters when you are sitting in the chair: the unasked question.
It matters most at the point of consent. Agreeing to something that cannot be undone, reducing a tooth for a crown or removing it altogether, assumes you understood the alternative, understood what happens if nothing is done, and asked about whatever was not clear. Consent you are translating in your head as you go is thinner than that, not because your understanding is poor but because part of your attention was spent on the language instead of on the decision. When the whole conversation runs in your first language, that attention goes back where it belongs.