Multi-language message support

[200sc]

Every message the engine outputs (as an error or otherwise) should be able to be localized to non-English languages as we get support for those languages. The act of translating our messages to other languages isn't going to happen any time soon realistically, but we should at least support the potential for this in the future.

Oak has a SetLang function but it isn't used for anything. The idea for this would be to move strings off to a subpackage, like oakerr but maybe with a different name (maybe oak/lang + oak/lang/oakerr) and add in support for 639-2 languages using the codes in https://godoc.org/golang.org/x/text/language.