The language into which the text should be translated. Options are:
BG - Bulgarian, CS - Czech, DA - Danish, DE - German, EL - Greek,
EN - English (unspecified variant for backward compatibility; please select EN-GB or EN-US instead),
EN-GB - English (British), EN-US - English (American), ES - Spanish, ET - Estonian,
FI - Finnish, FR - French, HU - Hungarian, ID - Indonesian, IT - Italian,
JA - Japanese, KO - Korean, LT - Lithuanian, LV - Latvian, NB - Norwegian (Bokmål),
NL - Dutch, PL - Polish, PT - Portuguese (unspecified variant for backward compatibility; please select PT-BR or PT-PT instead),
PT-BR - Portuguese (Brazilian), PT-PT - Portuguese (all Portuguese varieties excluding Brazilian Portuguese),
RO - Romanian, RU - Russian, SK - Slovak, SL - Slovenian, SV - Swedish,
TR - Turkish, UK - Ukrainian, ZH - Chinese (simplified).
assertthat::is.string(target_lang)