How to SEO on different local by Opencart?

Hi,

My shopping cart platform is opencart.

I am leaving at Vietnam, and i want to SEO my website for US ( it is okay now ), DE, JP, FR.
At current now, I am using multiple language on 1 store ( not multiple stores ).

Which thing i need to do now ?

1. Keep the current setting
2. Use multiple stores setting on 1 opencart ? Per store, i will use lang iso code for it? /de /fr /jp after URL.

Please give me an advice