add bcp47 strings normalisation

[zeroxbt]

[zeroxbt]

An easier and more efficient solution would be to add bcp47 normalization to the nquads parser. We could fork the jsonld library and add these changes :

https://github.com/zeroxbt/jsonld.js/commit/04f3a09d71d31e9ce7a28213ed0d3b3fc9a297b8