Add machine-translated README files for Japanese, Korean, Spanish,
French, German, Portuguese (BR), Turkish, Arabic, Hindi, and Russian.
Update language selector in English and Chinese READMEs to link all 12
versions.
New files: README.{ja,ko,es,fr,de,pt-BR,tr,ar,hi,ru}.md
Modified: README.md, README.zh-CN.md (language selector bar)
