Concurrent translations

Yan,

Here are translations for the Concurrent section.  I'll take a look at the Practice section next, then investigate localization techniques to better keep translations in sync.

I've added `.idea` and `.idea_modules` to `.gitignore`, I hope that's okay.

Also, I've marked a missing Chinese translation with `// Needed Chinese Translation below`. I'll go back through after the Practice section and see if there are any other missing translations.

Thanks!