Dear Sir/Madam,
It's my pleasure to submit this proposal.
I believe what I saw in the job detail is what my ability fits, so I'm ready to use my ability to help you finish this work. I've been a part-time translator for a long time, and I'm major in both Japanese and English now, so I believe I can help with you and get it done in the way you want.
And it will be my pleasure if we can have further collaboration.
Please let me know more details about this job.
Best wishes.