private static String checkLanguage(String lang) throws PropertyException {
        String inputLang = lang != null ? lang.toLowerCase() : "java";
        if (! legalLanguages.contains(inputLang)) {
            throw new PropertyException
                    ("Unsupported code language: '" + inputLang + "'. Supported languages are: " + legalLanguages);
        }
        return inputLang;
    }
Encapsulates data extracted from system properties.

@author gjoranv
configgen/src/main/java/com/yahoo/config/codegen/MakeConfigProperties.java