Improve multi-language support

[choldgraf]

Right now Solution to and Exercise are hard-coded (I think?). We should make it possible for these to be translated to new languages depending on the set language of the Sphinx config

[yves-chevallier]

I have the same issue for French support. I'd like to have Exercice and Solution de

It would be easy to use the sphinx language config and use i18n for different languages.