Sometimes a service provider has a language in common with their clients who are not fluent in the dominant language in that area. A common scenario is where a US-based provider knows Spanish on top of English. It is perfectly fine to speak that language to the client—it builds trust, eliminates the need for additional participants, and empowers the non-dominant language speaker. At the same time, it may still be a good idea to have an interpreter as part of your team.
First, not every bilingual speaker knows specialized vocabulary in both their languages. That does not mean that their language skills are somehow deficient. However, a proficient speaker of any language is unlikely to pick up specialized legal, medical, or engineering vocabulary without purposely studying it. Many polyglot professionals who got their degrees in the US admit that they can only discuss their work in English because that is the language they learned the terms of their trade in. If that is your case, an interpreter will help you with the business part of your conversation, while you may still engage in small talk and rapport-building.
Naturally, if you have the knowledge of your professional area in both languages, you can serve your client directly in the language of their preference. However, if other parties who do not speak the client’s language are present, an interpreter will make sure they are not excluded from the exchange. For example, your business partner or assistant should be able to follow the conversation and contribute to it if they so choose. Similarly, if you have a discussion with your associates in front of the client, the client should have access to that conversation. Everyone gets to speak the language they are most comfortable with, and the interpreter makes sure no one is left out.
In other words, an interpreter will let you focus on your job, without worrying about summarizing what was said for one of the parties who could not follow the conversation. An untrained bilingual speaker might gloss over some details or leave out important information, whereas trained interpreters strive to make the conversation accessible to all participants. That is why service providers who speak their client’s language should still consider using an interpreter if speakers of other languages are present.