The "most Kotlin-esque" way I found to do this sort of formatting is:

"%,.2f".format(Locale.GERMAN, 1234.5678)      // => "1.234,57"
"%,.2f".format(Locale.ENGLISH, 1234.5678)     // => "1,234.57"

"%,.2f".format(1234.5678)                     // => "1,234.57" for me, in en_AU

Note though that even though this is Kotlin's own extension method on String, it still only works on the JVM.

For those looking for a multiplatform implementation (as I was), mp_stools is one option.

Example:

import java.text.DecimalFormat;
import java.text.DecimalFormatSymbols;
import java.util.*;  

public class AddZeroToOneDigitDecimal{

     public static void main(String []args){
       System.out.println(customFormat(3434));
       System.out.println("----------");
       System.out.println(customFormat(3434.34));
       System.out.println("----------");
       System.out.println(customFormat(3434.3));
     }

     public static String customFormat(double d){

        String result =  formatter.format(d);
        return (result.replace(".00",""));
     }
     
     private static DecimalFormat formatter;
     private static final String DECIMAL_FORMAT = "#,###,##0.00";
     private static DecimalFormatSymbols formatSymbols = new DecimalFormatSymbols(Locale.ENGLISH);
    
     static {
         formatSymbols.setDecimalSeparator('.');
         formatSymbols.setGroupingSeparator(' ');
         formatter = new DecimalFormat(DECIMAL_FORMAT, formatSymbols);
     }
}

Simplified

 val formatter = DecimalFormat("#,###,##0.00");
 return formatter.format(value).replace(".00","");