Merge branch 'optional_introduction_in_pdf' into develop
[#65553094] added locale switch
[#65553094] pass locale to backend and reload dynamic content
[#65553094] dummy translations
[#65553094] fixed the locale switching links text
[#65553094] pass locale to custom downloads
[#37746913] another attempt at fixing this amazing bug
removed the appendix model and replaced previusly loaded appendices with a plain object array
[#65555794] load translated static pages asynchronously
[#65553094] workaround the error with passing search params during locale switch
[#65553094] removed locale from filters controller, use global var
[#65553094] pass locale to full downloads