OAI: elg crosswalk - langUtil with real iso 693 data, get shortest lang id

[milanmajchrak]

• There is also missing whole elg.xsl crosswalk: Original issue: Missing elg crosswalk ufal/clarin-dspace#1029
• There is some schema which prefers 2 letters code instead of 3 letters..

[kosarko]

ufal-showstopper potrebujeme elg crosswalk

@vidiecan trocha kontextu: elg jako schema je nastupce metashare

[milanmajchrak]

@Paurikova2 Write Python tests to test this crosswalk