Add translations of "7 Principles of Rich Web Applications".

[leerob]

This PR adds Japanese and Russian translations for 7 Principles of Rich Web Applications using Nextra's built-in i18n support.

• Added Japanese translation from this source
• Added Japanese translation from this source

What's Left?

• Still need to do the Portuguese translation
• Might want to have others who speak these languages proofread the translations

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