Keyword localization

I'll preface this by saying I'm a native english speaker and have never lived through the experience of trying to program in a language that assumes anything but english fluency. I'm certainly not the right person to plan such a feature, but I want to float the idea in case any non-native english speakers want to weigh in on opportunities for improvement on english-first languages and how they might be addressed.

The [history of localized languages](https://en.wikipedia.org/wiki/Non-English-based_programming_languages) seems to indicate that there is at least some interest for this, and I'm very interested in experiments for a more native implementation. I'd like to explore translations early so that they're a central feature of the language.

Although I think it's important that there is an assumed language for the portability of code, there could certainly be a way to opt-in to swap out the grammar's keywords and condition message translations. The idea is that a language might have alternative language modes (`r --lang=es`) that would allow for localized keywords:

```r
#! usr/bin/env r # #! usr/bin/env r --lang=es
f <- function(...) { # f <- funcion(...) {
if (TRUE) { # si (CIERTO) {
} else { # } sino {
} # }
} # }
```

(apologies to any spanish speakers if this isn't a sensible translation!)