Add error message translations

[Bisaloo]

One big benefit of checkmate over checking your argument via base R is that the error message is automatically created & standardised.

I believe providing translation of error messages would be an extra argument to adopt this package as a dependency as it would make my package multilingual out-of-the-box (vs base R where I would have to translate each error message myself).

By translating once here, you potentially remove this need for many dependent package developers.

[Bisaloo]

We still have strong interest in this because we use checkmate across packages that are targeted to users in English-, Spanish-, Portuguese-, and French-speaking countries, among others.

We would happily submit a PR with the required changes, a detailed explanation of the translation process (via the potools R package), and we would help to the maintenance of this feature at least for the next couple of years.

Would you be interested in such a PR?