Why the translator is expected to get Payment after 90 days?
1. I do not know whether this is normal. I know, that each project you handle is unique, each agency is different. May be you know the agency. You are working for them for long and getting payments in time. SO you may be perfectly comfortable in accepting their payment terms of 90 days.
2. But I think it is not OK. If I were you, I would NOT accept this term because 90 days is too long a period. May be 45 days is OK.
3. It may sound very tempting that you are promised assignments week after week. However, what if they don't pay you as promised. That means, they owe you more and more money. If by any chance agency decides not to pay you would have to spend more time go trying to get your money. And time is money.
4. If you accept ninety days, please don’t not take on any additional assignments from them; instead, tell them that you will be available again as soon as they have settled all outstanding accounts.
5. There are a number of agencies in who reasons that their customers don't pay them. It is not logic. You do not have any agreement with the customers of your client. Your agreement is with your direct client.