Do the staff in the office for foreign labour speak English? My experience in Bangkok is that in general English might not be their strongest skill - although you might hit it lucky and find a very good English speaker. Of course the staff could always reverse the question and ask how long it will be until you speak Thai.

In addition to the above you will need a completed application form and I suspect they will want a job description as well. These forms need to be completed in Thai.

The only way to know for sure is to contact your labour office and ask.

You will be applying for the work permit not just the letter of approval. WP1 application package that lists the basic requirements is here: http://wp.doe.go.th/wp/images/form/Form_tt1.pdf