Translate missing lines - 1.5.5.1 to 1.5.6

At the moment, my language is translated only up to v1.5.5.1. I would like to translate missing texts and make it compatible with v1.5.6. I was wondering how can I do that?

Here is my idea: I would somehow need to merge English and my language together by inserting missing lines from English translation files into the current local ones, or vice versa. What's the easiest way to do that?

the easiest way is to open each file one by one to do so