Map<String, String> tm = new Map<String, String>{
'Monday'=>'u041fu043eu043du0435u0434u0456u043bu043eu043a',
'Tuesday'=>'u0412u0456u0432u0442u043eu0440u043eu043a',
'Wednesday'=>'u0421u0435u0440u0435u0434u0430',
'Thursday'=>'u0427u0435u0442u0432u0435u0440',
'Friday'=>'u041f'u044fu0442u043du0438u0446u044f',
'Saturday'=>'u0421u0443u0431u043eu0442u0430',
'Sunday'=>'u041du0435u0434u0456u043bu044f'};
System.debug(LoggingLevel.ERROR, '@@@ v: ' + String.join( new List<String>(tm.keySet()), ',') );
System.debug(LoggingLevel.ERROR, '@@@ v: ' + tm.keySet() );

Map<String, String> currencyMap = new Map<String, String>();
currencyMap.put('France','Euro'); currencyMap.put('Japan', 'Yen');
// Iterate through the map elements.
for (String mapKey : currencyMap.keySet()) {
            System.debug('Key: ' + mapKey +
                ', Value: ' + currencyMap.get(mapKey)); }

        // The output is: Key: France, Value: Euro Key: Japan, Value: Yen