I don't mean to bother you but could you let me know when I can get offer letter please?

I'm asking because I have got a offer letter from another company and I would feel more comfortable if I can decline that offer after I get official offer letter from your company.

The words you use are quite direct, which is not wrong, but being more indirect will seem more respectful, especially in this case that you want the job from the company. If I were you, I would put the email in this way: 

Email title: Enquiry about job application status

Dear Sir/Madam,

Thank you for the interview opportunity. May I know are there any updates available with regards to my application for the job as [job title]? I received an offer from another company, but [company name] is my first priority, so I would be grateful if I could have the opportunity to work in  your company.

You could contact me at [your contact information]. Thank you in advance for your prompt reply.

Best regards,

[Your name]

>>>>>fill in the information in the brackets [  ]  yoruself

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Business email - asking when I can get offer letter

I'm asking because I received an offer letter from another company and I would feel more comfortable if I can decline that offer after I receive the official offer letter from your company.