Add support for multiple languages

[roata]

Some people requested a Russian UI.

• figure out which localization tool to use for strings, perhaps Babel?
• find a privacy-preserving way to store a user's language preferences
• ensure the strings are factored out into separate resources, to make translations by contributors easy

[iamandrewluca]

If someone wants to contribute to translating to different langauges we can use
https://crowdin.com/
https://crowdin.com/page/open-source-project-setup-request

[ralienpp]

A privacy-preserving way to store the user preferences would be to hash the name+username string, and use that as a key in key-value store.

[roata]

Andrew suggested the use of the language_code attribute of a user profile:
https://core.telegram.org/bots/api#user

This is the way to go.