French French French! I suddenly want to start to learn French! Anybody could help? :3

A Business Letter. The article below I write is a reply to a letter of complaint. This is my first time to write such a formal letter, so I would need your help to tell me if this is eligible, which means if the way I talk in the letter is formal enough and whether the reason I give in the letter is acceptable or not. Oh, and please also correct the grammar mistakes. ._. Thank you all in advance! :) Dear Sir, In response to your letter of complaining about the temporary administrative assistant our company sent you who didn’t meet your satisfaction, I would like to apologize. I am writing this letter to explain to you how this all happened. Here is the reason why this unhappy thing happened. You may not know, our employees are sorted into different groups according to the various needs of our costumers. All of the employees have to been trained at the same when they join our company. We keep expanding the amount of our employees to meet the growing demand in the market. Unfortunately, due to the fact that administrative assistants are in badly need these days, the experienced old staff had all been occupied before you asked for one. After we thought repeatedly, we decided to give the newly recruited employee who is still under the training this opportunity. We must apologize for taking it lightly. Our company has been in operation for 3 years already and have a very good reputation for sending satisfactory temporary workers quick with accuracy. I believe that's why you chose ask and allow me to show my great thankfulness for your trusting us. I beg your understanding and forgiveness since this is the first time that this kind of case happens and I can promise that this would never happen again. We would be very appreciated if you could give us a second chance by taking our compensation. We will send you a temporary assistant, free of charge, when you need once the next time. With apologies once again. Yours faithfully, XXX