In the current WikibaseLexeme Lua, lexemes have getLemmas(), forms have getRepresentations(), and senses have getGlosses(), all of which return a plain list of value+language pairs. Methods to get the value by language code would probably be more useful: getLemma( languageCode ), getRepresentation( languageCode ), getGloss( languageCode ).
Returning the language code as a second return value (Lua!) would probably be useful for something like local lemma, language = lexeme:getLemma( complex.expression['for'].language.code ). If there is no value in the given language, return nil, of course.