English is not owned by Americans or British so no need to measure yourself to their standards. English is the lingua franca and is owned by all people of the world. Japanese speak in a certain way but that's fine as long as you can understand and be understood. I sometimes find with Japanese there is the belief that English not spoken with a perfect American accent is not good English but this is an impossible standard to reach for those not born and raised in the US. And some of the most interesting people speaking and writing in English today are non native speakers.