Setup internationalization framework

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This task is the first step of #3.
It includes:

• Find and choose an internationalization library (there are many e.g. https://github.com/i18next/react-i18next https://github.com/yahoo/react-intl )
• Set it up to read the default language from OS via Electron.
• Add a setting option for users to change the language

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What's left to do on this issue, is to set up languageDetector: https://github.com/i18next/i18next-browser-languageDetector