You would be using mainly English in this job. Do you think you could manage?
Yes, I am sure I can.
Do you think you are proficient in both written and spoken English?
Yes, I think I am quite proficient in both written and spoken English.
Do you think you can make yourself understood in English with ease?
Yes, I think I can in ordinary circumstances.
Do you think you can speak English quite fluently? Tell me about your English education.
Yes, I think I speak English quite fluently. I got a high score on TOEFL. I majored in English in college.