Is an English translation of the letter publicly available, and where can I find it?
If you are talking about finding it online, you aren't going to have any luck. These letters, as far as I know, are not available to read online.
However, if you are willing to spend some money, an English book compilation of the letters by George Mastrantonis can be bought from Amazon (published by Holy Cross Orthodox Press, however it seems to be more expensive on their website).
Quoting a review on Amazon by Dusk:
This book is very meaty, but I recommend it strongly to those interested. The letters provide a deeper understanding of both Lutheran and Orthodox beliefs. They provide a window into the age they were written in: the manners used in disagreement are delightful. They also provide a look into human nature itself. Read carefully or you may injure yourself.
I hope this helps! Have fun...