The researcher also will give questionnaire to the students and interview them to measure their motivation FOR / IN learning English.
We appear to have two possible meanings... "Why are you learning English?" (measure their motivation for learning English), and "How hard are you trying to learn English?" (measure their motivation in learning English). Since the word used is "measure", which usually refers to determining a value out of a range of values, rather than "determine" or "assess" the motivation, I would choose "IN" here. However, others might use "FOR" to mean the same thing. The question is ambiguous, which is probably why it hasn't been answered in four years.