Extract Strings from Flutter / Dart Project. Also if you are using Easy Localization, you can add one translation at a time to all json files.

add_lang.dart

import 'dart:io';
import 'dart:convert';

void updateLocalizationKeyInLanguages(
    List<String> filePaths, String key, Map<String, String> translations) {
  try {
    for (String filePath in filePaths) {
      File file = File(filePath);

      if (file.existsSync()) {
        String content = file.readAsStringSync();
        Map<String, dynamic> jsonData = json.decode(content);

        // Update the value for the specified key with translations for each language.
        // print(filePath.split('.').first.split("/")[2]);
        jsonData[key] = translations[filePath.split('.').first.split("/")[2]];

        String updatedContent = json.encode(jsonData);
        file.writeAsStringSync(updatedContent);

        print('Updated "$key" in $filePath.');
      } else {
        print('File not found at $filePath');
      }
    }
  } catch (e) {
    print('Error updating JSON files: $e');
  }
}

Future<List<String>> loadJsonFilesFromAssets() async {
  final Directory directory = Directory("assets/translations");
  final List<String> jsonFiles = [];

  if (await directory.exists()) {
    final List<FileSystemEntity> files = directory.listSync();

    for (var file in files) {
      if (file is File && file.path.toLowerCase().endsWith('.json')) {
        jsonFiles.add(file.path);
      }
    }
  }

  return jsonFiles;
}

void main(List<String> arguments) async {
  if (arguments.length < 2) {
    print('Usage: dart script.dart <keyToUpdate> <jsonDirectory>');
    return;
  }

  String jsonKey = arguments[0];

  String keyToUpdate = 'newsletters';

  // Translations for the "update" key in different languages.
  Map<String, String> translations = {
    "en": "Newsletters",
    "de": "Newsletter",
    "nl": "Nieuwsbrieven",
    "es": "Boletines",
    "pt": "Boletins informativos",
    "ar": "النشرات الإخبارية",
    "ru": "Новостные бюллетени",
    "es-419": "Boletines"
  };

  final jsonFilePaths = await loadJsonFilesFromAssets();

  if (jsonFilePaths.isNotEmpty) {
    updateLocalizationKeyInLanguages(jsonFilePaths, keyToUpdate, translations);
  } else {
    print('No JSON files found in the specified directory.');
  }
}

string_extractor.dart

import 'dart:io';
import 'dart:convert';

void main() {
  final extractedStrings = <String, String>{};

  // Replace 'lib' with the path to your Flutter project's source code directory.
  final projectDirectory = Directory('lib');

  _scanDirectory(projectDirectory, extractedStrings);

  final sanitizedStrings = <String, String>{};

  extractedStrings.forEach((key, value) {
    final sanitizedKey = sanitizeKey(key);
    sanitizedStrings[sanitizedKey] = value.contains("-") ? value.replaceAll("_", " ") : value;
  });

  final outputFile = File('strings.json');
  outputFile.writeAsStringSync(jsonEncode(extractedStrings), flush: true);
}

String sanitizeKey(String key) {
  // Convert to lowercase and replace spaces with underscores.
  return key
      .replaceAll(RegExp(r'[^a-z0-9]'), '_')
      .replaceAll(RegExp(r'^_+|_+$'), '').toLowerCase();
}

void _scanDirectory(Directory directory, Map<String, String> extractedStrings) {
  final files = directory.listSync();

  for (final file in files) {
    if (file is File && file.path.endsWith('.dart')) {
      final content = file.readAsStringSync();
      final stringLiterals = RegExp(r"'(.*?)'").allMatches(content);

      for (final match in stringLiterals) {
        final extractedString = match.group(1);
        if (extractedString != null &&
            !extractedString.contains('package:') &&
            !extractedString.contains('dart') &&
            !extractedString.startsWith('https') &&
            !extractedString.contains('\$') &&
            !extractedString.contains('getIt<') &&
            !extractedString.contains('AppUrl') &&
            !extractedString.contains('/') &&
            !extractedString.contains('\\')) {
          extractedStrings[extractedString] = extractedString;
        }
      }
    } else if (file is Directory) {
      _scanDirectory(file, extractedStrings);
    }
  }
}