求翻译：买方应由卖方可接受的银行于装船月份前30天开立并送达卖方不可撤销即期信用证，至装运月份后第15天在中国议付有效。是什么意思？

The buyer by the seller bank acceptable to the opening of the first 30 days of shipment month and delivered to the Seller an irrevocable sight letter of credit to 15 days after the month of shipment for negotiation in China.

Buyer shall be accepted by the seller may ship in the bank for 30 days before the opening and was given to the seller that an irrevocable letters of credit, for shipment to the first 15 days following the month in the conference in China pay effective.

正在翻译，请等待...

Bank acceptable to the buyer by the seller to ship 30 days before the opening of the month and served on the seller an irrevocable letter of credit, to the ship on the 15th day of the month in the Chinese negotiating effectively.

The buyer should the bank which may accept by the seller previous 30 days draw up and deliver the seller in the shipment month not to be possible to abolish the immediate letter of credit, 15th days is effective after the shipping month in the Chinese negotiation.