Detect language unambiguously, or raise explanatory exception

[pjkundert]

Correctly and unambiguously determine Mnemonic language.

@prusnak, perhaps this is sufficiently clear that it doesn't make the reference implementation less tidy?

[prusnak]

Concept ACK

@matejcik What do you think about the approach?

[matejcik]

concept ACK but please fix the style check

[pjkundert]

OK, @matejcik, I've fixed the style_check issues (the remaining failures are unrelated to this pull)

[matejcik]

the style check is still failing for the test file

[matejcik]

ah but it's something that was in the repo already. I'll fix that.