How can the web interface be translated into other languages?

This is not possible with the current version of Ubooquity

But the next version (Ubooquity 3) will allow full control over the html pages with a new theme system, so you will able to translate the text you want.

I will release a beta version this summer.

Thank you for answering so quickly, if you need any help with the translation into Spanish, 
Catalan or Galician, Basque or French, do not hesitate to ask, I will be happy to help you.