Actually, in many areas of the public sector, it's considered good practice to have interpreting services available and free at the point of use for people whose first language is not English, even if they have some understanding of English.
In healthcare, for example, providing a medical history in your 2nd, 3rd or even 4th language isn't going to be nearly as detailed as it would be in your native language, and there can be culturally distinct elements to how someone presents - missing those means a less thorough and accurate assessment.
In law, it's much harder to follow legal proceedings in a 2nd, 3rd, 4th language. Having access to an interpreter helps counterbalance those disadvantages.
As I've said before, it's hard to learn and communicate in multiple languages. Using an interpreter doesn't mean someone hasn't tried.