日本で捕まえられるポケモンをコンプリートしたって英語でなんて言うの？

There are no Pokemon left in Japan for me to find - means that you found every single one of them.

These sentences explain that you have completed the game. There are no more for you to find or catch.

These are two simple ways to say this.

"I caught all the available in Japan" is easy to understand.

1. This phrase states the facts.
2. 'As far as _ goes... = In the case of __
3. 'em is an abbreviated form of 'them'. It represents the spoken word where the 'th' sound is often lost through elision - the contraction of words in their spoken form.

We can use the sentences above to explain that you have collected all Pokemon in Japan playing Pokemon Go.

"I enjoy this game so much that I have caught all the Pokemon that exist in Japan."
This sentence explains that your love for the game helped you find all the Pokemon available in your country.

All three sentences explain to the listener that you have caught all the Pokémon that was on the catch list issued by the game for Japan. All three sentences are stated facts of something or some action that you did which is related to the game you play.
You will use the first sentence when a person doesn't know the game and is uncertain of what you are saying to them.

*uncertain - not sure