I have written a fictional novel of about 40,000 words with a very bad translation to English. I need someone to expand it with 30,000 more words to reach 70,000 and become a partner with me for publishing it. Where I can find a person?
You are looking for a ghostwriter. In your search engine of choice, search for "ghostwriter wanted" or "where to find a ghostwriter" or variations thereof to find where ghostwriters are commonly found. You will have to pay that person, of course – no one will write your book for you for a share in the potential profits.
Finding a writing partner is hard, and finding a co-author is even more difficult.
You find writers in writers' communities. Most large cities have many writer's meetups and public writing groups. For instance, near me in Denver, CO, there are dozens of public groups where writers come together in person to share their work and share feedback. There are many times many groups that meet online -- via remote meeting places like zoom or over email.
Join as many communities in your genre and near your skill level and meet as many writers as you can. You'll find some that you click with and some that click with you. That pool of writers is your best chance for finding a co-author short of hiring a ghostwriter -- as has been recommended -- or hiring an editor to help you develop your story into something publishable.