I am an interpreter and business assistant. How can I ask for payment if there is no written agreement beforehand? What can I say to my client?
As side comment - I really wonder about some of the questions that have been sent here the last days.
To answer your question: this is called communication. You send them an invoice. If they do not pay, yo utake them to court. There is also no written agreement if you order something in a restaurant, you know.
Your accountant will gladly enlighten you on the particulars of what is required legally for an invoice in your jurisdiction. And yes, going along without consulting an accountant beforehand is utterly stupid - you are bound to get into tons of trouble, especially given the fact that you obviously are a little clueless about basics business operations.
Use one of the job sites like elance or vworker. They'll collect the money up front and act as an escrow agent. There sites that specialize in translations. Haven't used them, but I suspect their FAQ will answer your questions.
This is generally easy if your credibility is high. If its not, try getting it higher (by making a good looking site, participating on social sites and forums etc). You don't have to make a written agreement, you're not doing a million dollar deal. You should ask for a small advance fee and then the complete fee.
You can also just make a sample of what you are making (for example: if I am making a web design, I can make a mock-up of the layout which shows where everythings going to be without color or details). If they like it, they give you advance fee (keep is like 10-20% of your fee) and start working.
You can also add testimonials (though most online ones tend to be fake). You can try gaining their trust by referencing your previous customers but thats only if they were satisfied with you.
I recommend you read this book, it deals with exactly this.. "Gaining credibility". Its pretty cheap.
you can also check out all their products, they help you a lot.
I'm assuming you've been paid before, but may have some clients who are attempting to steal your services. Ask why they feel you did not fulfill your side of the agreement. I don't know about your country, but in the US you can take them to court. Hopefully it won't come to that.
If you feel this is a problem, you need to get something in writing beforehand.