I wanted to retrieve the details using the Metadata API. Anyway, you can use the below code:

List<String> labels = new List<String>{'Demo1'};

MetadataService.MetadataPort service = new MetadataService.MetadataPort();
service.SessionHeader = new MetadataService.SessionHeader_element();
service.SessionHeader.sessionId = UserInfo.getSessionId();

Map<String, String> valuesByKey = new Map<String, String>();
MetadataService.Translations translations = (MetadataService.Translations) service.readMetadata('Translations',
                new String[] { 'en_US' }).getRecords()[0];
for(MetadataService.CustomLabelTranslation customLabelTranslation : translations.customLabels) {
    if (labels.contains(customLabelTranslation.name)) {
        valuesByKey.put(customLabelTranslation.name, customLabelTranslation.label);
    }
}
System.debug(valuesByKey);

I retrieved the details using the apex-mdapi. Example code below:

List<String> labels = new List<String>{'Demo1'};

MetadataService.MetadataPort service = new MetadataService.MetadataPort();
service.SessionHeader = new MetadataService.SessionHeader_element();
service.SessionHeader.sessionId = UserInfo.getSessionId();

Map<String, String> valuesByKey = new Map<String, String>();
MetadataService.Translations translations = (MetadataService.Translations) service.readMetadata('Translations',
                new String[] { 'en_US' }).getRecords()[0];
for(MetadataService.CustomLabelTranslation customLabelTranslation : translations.customLabels) {
    if (labels.contains(customLabelTranslation.name)) {
        valuesByKey.put(customLabelTranslation.name, customLabelTranslation.label);
    }
}
System.debug(valuesByKey);

I wanted to retrieve the details using Metadata API. Anyways, you can use the below code:

List<String> labels = new List<String>{'Demo1'};

MetadataService.MetadataPort service = new MetadataService.MetadataPort();
service.SessionHeader = new MetadataService.SessionHeader_element();
service.SessionHeader.sessionId = UserInfo.getSessionId();

Map<String, String> valuesByKey = new Map<String, String>();
MetadataService.Translations translations = (MetadataService.Translations) service.readMetadata('Translations',
                new String[] { 'en_US' }).getRecords()[0];
for(MetadataService.CustomLabelTranslation customLabelTranslation : translations.customLabels) {
    if (labels.contains(customLabelTranslation.name)) {
        valuesByKey.put(customLabelTranslation.name, customLabelTranslation.label);
    }
}
System.debug(valuesByKey);

I wanted to retrieve the details using the Metadata API. Anyway, you can use the below code:

List<String> labels = new List<String>{'Demo1'};

MetadataService.MetadataPort service = new MetadataService.MetadataPort();
service.SessionHeader = new MetadataService.SessionHeader_element();
service.SessionHeader.sessionId = UserInfo.getSessionId();

Map<String, String> valuesByKey = new Map<String, String>();
MetadataService.Translations translations = (MetadataService.Translations) service.readMetadata('Translations',
                new String[] { 'en_US' }).getRecords()[0];
for(MetadataService.CustomLabelTranslation customLabelTranslation : translations.customLabels) {
    if (labels.contains(customLabelTranslation.name)) {
        valuesByKey.put(customLabelTranslation.name, customLabelTranslation.label);
    }
}
System.debug(valuesByKey);