Usilize i18n capabilities of Hugo

[irnc]

[back in 2015] Hugo doesn’t natively support i18n so I also had to do a few things to work around this.

When LearnOverpass was created, there was no support for i18n in Hugo. Content was split on language-based sections, which is not enough to have fully localized site as it leaves content in layout files untranslated.

Nowadays Hugo has multilingual mode, so we could use it for complete site localization.

The complete site localization goal is important as it would make LearnOverpass friendlier for non-English learners.