Use standard mechanisms to offer language options

[Bisaloo]

epiCo currently proposes the output in English or Spanish in a number of functions. This is great to improve usability & adoption!

I think the current way to specify language could be improved by using R standard mechanisms (via the gettext() function) and automatically detecting the user language.

Benefits:

• from the user point of view, no need to manually specify the value of the lang argument in numerous function calls
• from a maintainer point of view, reduction of the code duplication and streamlining of the addition of new translations

It may be good to have global meeting on this at some point so we can start adding multilingual options to many Epiverse-TRACE packages in a standard way.