Set up internationalization and translate the admin panel

[MrNaif2018]

We need to set up internationalization infastructure on admin panel.
Initially, translating only checkout page is enough. Later we can translate the whole panel.
Steps to do it:

• Set up nuxt/i18n. See this commit and further ones to see how we did it for bitcartcc store
• Setup transifex resource. I will do it when initial setup is done
• Start translating checkout page. The translation process should look the same as in bitcart/bitcart-site#86
• Try translating the whole admin panel. This would mean to change vuetify locale too, as it has some locales supported.