Well, it's simple - sort of. When the primary authors of the Catechism of the Catholic Church (6 Bishops and 1 Priest) were chosen, the language that they all had in common was French, so it was natural for them to collaborate and write in that language. You may remember that the Catechism was first published in French.
The French was then translated to English and we received the first English edition of the Catechism.
During this time the Church was also translating the French edition to Latin. This Latin edition became the official edition, known as the Editio Typica.
Once we had the Editio Typica, it became the task of translators to translate the Latin to English to give us a better English translation which was more true to the Editio Typica. this became known as the "Second English Translation" of the Catechism.
How do you know if you have the second edition?
1- If you have the big green catechism, you have the second edition.
2- If you have one of the smaller hardcover or softcover versions and it either says "Second Edition" or "Includes modifications from the Editio Typica" on it, then you also have the second edition.
This may not have been a burning question plaguing you, but it's pretty cool information to know!