In depth parsing of locales in Java

Parsing strings into locales is often used in projects, but there isn't a good class for it.
Java itself provides three constructors, but the actual use of the process, need to resolve their own, more cumbersome.
The Locale (langugae)
The locale (language, country)

Locale(String language, String country, String variant)
Apache has a localeutils.tolocale (String language) but does not support lowercase for the last two characters, such as zh_CN, which is not supported.

Referring to other methods, I wrote the following class and made a note of it, which I hope will be helpful to those who solve similar problems.

import java.util.Locale;
public class LocaleUtil
{
    private final static Locale DEFAULT_LOCALE = Locale.ENGLISH;

    public final static String IETF_SEPARATOR = "-";  

    public final static String SEPARATOR = "_";  

    public final static String EMPTY_STRING = ""; 

    
    public static Locale toLocale( String language )
    {
        if( !StringUtil.isNullOrEmpty( language ) )
        {
            return langToLocale( language, SEPARATOR );
        }
        return DEFAULT_LOCALE;
    }

     public static Locale langToLocale( String lang , String separator )
     {
           if( StringUtil.isNullOrEmpty( lang ) )
           { 
                return DEFAULT_LOCALE;
           }
            String language = EMPTY_STRING;
            String country =  EMPTY_STRING;
            String variant =  EMPTY_STRING;
            int i1 = lang.indexOf( separator );
            if ( i1 < 0 )
            {
                language = lang;
            } else 
            {
                language = lang.substring(0, i1);
                ++i1;
                int i2 = lang.indexOf( separator, i1);
                if (i2 < 0) 
                {
                    country = lang.substring(i1);
                } else 
                {
                    country = lang.substring(i1, i2);
                    variant = lang.substring(i2+1);
                }
            }

            if(language.length() == 2)
            {
               language = language.toLowerCase();
            }else 
            {
              language = EMPTY_STRING;
            }

            if(country.length() == 2)
            {
               country = country.toUpperCase();
            }else 
            {
              country = EMPTY_STRING;
            }

            if( (variant.length() > 0) && 
                ((language.length() == 2) ||(country.length() == 2)) )
            {
               variant = variant.toUpperCase();
            }else
            {
                variant = EMPTY_STRING;
            }

            return new Locale(language, country, variant );
        }
}

Note:

 public class StringUtil
{
    public static boolean isNullOrEmpty(String target) {
        return target == null || "".equals(target);
    }
}