You'll need to determine a unique identifier for the caller such as an account number. You might be able to use caller ID if it's a business-to-consumer call center; however, many business phones send the company's main number vs. the person's direct line as caller ID. This makes for a not-very-unique identifier when it happens. If you end up using caller ID you can get it with the Get Call Contact Info step.
Once you have it, you'll need to store it to a database (or web service front-end if you prefer) so that future calls can query the database to see if a language has been set for that unique ID.
Please remember to rate helpful responses and identify helpful or correct answers.
