If English is not your mother tongue, you may be wondering that, if you run into a job interviewer who shares the same mother tongue as you, whether you can conduct the interview in your common ancestral language. This is a completely legitimate question, especially as most quants nowadays come from countries that do not have English as an official language.
The short answer is, you shouldn’t. The long answer, as always, is a bit complicated.
In most cases, you shouldn’t even ask the interviewer if you two can speak your (shared) native language. Many interviewers, including those born in foreign countries, consider conducting an interview in a language other than English unprofessional and inappropriate. Some interviewers don’t want to leave you the impression that she or he will favor you over other candidates because you two share a common heritage. A few people may even be ashamed of their “old country” so that they would hate your idea of conversing in the “old country language.” Finally, it’s possible a person who seems to be from your old country may not actually be able to speak the national language, or your particular dialect.
So, don’t risk your candidacy. Stick to English regardless of your own comfort level with the language and regardless of the interviewer’s ethnic or national background.
The only exception to this rule is if the interviewer asks you if you’d like to have the interview in your native tongue, and you are very sure he or she is sincere in this offer. If you can’t be sure whether he or she is sincere or trying to “bait” you into doing something stupid, politely decline and insist on speaking English throughout the interview, but you can have some small talk afterwards in your native language.