Expose support for i18n/translation in the public C++ API

[veloman-yunkan]

libkiwix server component has been internationalized a while ago, but the support for that functionality has not been exposed in the public C++ API.

kiwix/kiwix-desktop#972 has now put enough pressure on libkiwix to finally start providing translation services to its C++ clients.

[kelson42]

@ShaopengLin We really need to move forward on this!

[ShaopengLin]

I am waiting for veloman to respond to my comment here about filtering. It also affects what I need to do here.

I can put up a priliminary version for now then.

Resolved.