Support internationalization on vuejs based interface

As part of #109 (closed) we want to have full i18n support for the VueJS based UI.

This issue surpasses #171 (closed), so please have a look at it, as it contains some useful information for discussion.

Open questions:

• How to extend the current i18n support to the VueJS files?
  • Will we be able to use the vue-i18n component?
  • How well does it integrate with the current approach (i18n being done in the server side)?
  • if not, how can we expose the contents of the .po files to VueJS?
• How to integrate it to our current tooling to extract strings to generate .po files?
• If gettext and VueJS are incompatible and we decide for using some pure VueJS i18n support, how can we integrate it to our translation workflow (weblate)?

For research:

• https://github.com/Polyconseil/vue-gettext
• https://kazupon.github.io/vue-i18n/