Once you learned Japanese, you will find that English is easy to learn.
You'll feel that it is easy to learn English when you had learned the Japanese..
一旦日本语を勉强したら、必ず英语の易しさが感じられるのです。
You will find that English is much easier once you learn Japanese.
Once you learn Japanese ,you will find English is easier
Once you have learned Japanese, you will find English easier.