diff --git a/i18n_key_sync/locale.py b/i18n_key_sync/locale.py
new file mode 100644
index 0000000..2927539
--- /dev/null
+++ b/i18n_key_sync/locale.py
@@ -0,0 +1,222 @@
+"""Locale file parsing for JSON and YAML format."""
+
+import json
+from pathlib import Path
+from typing import Any, Dict, List, Optional, Set
+
+
+class LocaleParser:
+    """Parse and manage locale files in JSON and YAML formats."""
+
+    def __init__(self):
+        """Initialize the locale parser."""
+        self._cache: Dict[str, Dict[str, Any]] = {}
+
+    def parse_file(self, file_path: Path | str) -> Dict[str, Any]:
+        """Parse a single locale file.
+
+        Args:
+            file_path: Path to the locale file.
+
+        Returns:
+            Dictionary of locale keys and values.
+
+        Raises:
+            ValueError: If the file format is not supported.
+            FileNotFoundError: If the file doesn't exist.
+
+        """
+        path = Path(file_path)
+        path_str = str(path)
+        if path_str in self._cache:
+            return self._cache[path_str]
+
+        content = path.read_text(encoding="utf-8")
+        suffix = path.suffix.lower()
+
+        if suffix == ".json":
+            data = json.loads(content)
+        elif suffix in [".yaml", ".yml"]:
+            import yaml
+
+            data = yaml.safe_load(content)
+        else:
+            raise ValueError(f"Unsupported locale file format: {suffix}")
+
+        if not isinstance(data, dict):
+            data = {}
+
+        self._cache[str(file_path)] = data
+        return data
+
+    def get_all_keys(self, data: Dict[str, Any], prefix: str = "") -> Set[str]:
+        """Get all keys from nested dictionary, including nested keys as dot notation.
+
+        Args:
+            data: Nested dictionary of locale data.
+            prefix: Prefix for nested keys.
+
+        Returns:
+            Set of all keys in dot notation.
+
+        """
+        keys = set()
+        for key, value in data.items():
+            full_key = f"{prefix}.{key}" if prefix else key
+            if isinstance(value, dict):
+                keys.update(self.get_all_keys(value, full_key))
+            else:
+                keys.add(full_key)
+        return keys
+
+    def flatten_keys(self, data: Dict[str, Any], prefix: str = "") -> Dict[str, Any]:
+        """Flatten nested dictionary to dot notation.
+
+        Args:
+            data: Nested dictionary of locale data.
+            prefix: Prefix for nested keys.
+
+        Returns:
+            Flattened dictionary with dot notation keys.
+
+        """
+        flattened = {}
+        for key, value in data.items():
+            full_key = f"{prefix}.{key}" if prefix else key
+            if isinstance(value, dict):
+                flattened.update(self.flatten_keys(value, full_key))
+            else:
+                flattened[full_key] = value
+        return flattened
+
+    def unflatten_keys(self, data: Dict[str, Any]) -> Dict[str, Any]:
+        """Unflatten dot notation keys to nested dictionary.
+
+        Args:
+            data: Flattened dictionary with dot notation keys.
+
+        Returns:
+            Nested dictionary.
+
+        """
+        nested: Dict[str, Any] = {}
+        for key, value in data.items():
+            parts = key.split(".")
+            current = nested
+            for part in parts[:-1]:
+                if part not in current:
+                    current[part] = {}
+                current = current[part]
+            current[parts[-1]] = value
+        return nested
+
+    def parse_directory(
+        self,
+        dir_path: str,
+        locales: Optional[List[str]] = None,
+    ) -> Dict[str, Set[str]]:
+        """Parse all locale files in a directory.
+
+        Args:
+            dir_path: Path to the locale directory.
+            locales: Optional list of locale names to include.
+
+        Returns:
+            Dictionary mapping locale names to their key sets.
+
+        """
+        locale_dir = Path(dir_path)
+        results: Dict[str, Set[str]] = {}
+
+        if not locale_dir.exists():
+            return results
+
+        for locale_file in locale_dir.iterdir():
+            if not locale_file.is_file():
+                continue
+
+            locale_name = locale_file.stem
+            if locales and locale_name not in locales:
+                continue
+
+            try:
+                data = self.parse_file(locale_file)
+                keys = self.get_all_keys(data)
+                results[locale_name] = keys
+            except (ValueError, json.JSONDecodeError) as e:
+                if locales:
+                    raise ValueError(f"Error parsing {locale_file}: {e}")
+
+        return results
+
+    def write_file(
+        self,
+        file_path: Path,
+        data: Dict[str, Any],
+        format: str = "json",
+    ) -> None:
+        """Write locale data to a file.
+
+        Args:
+            file_path: Path to the output file.
+            data: Dictionary of locale data.
+            format: Output format (json or yaml).
+
+        """
+        content: str
+        if format == "json":
+            content = json.dumps(data, indent=2, ensure_ascii=False)
+        elif format in ["yaml", "yml"]:
+            import yaml
+
+            content = yaml.dump(data, default_flow_style=False, allow_unicode=True)
+        else:
+            raise ValueError(f"Unsupported format: {format}")
+
+        file_path.write_text(content, encoding="utf-8")
+
+    def add_key(
+        self,
+        data: Dict[str, Any],
+        key: str,
+        value: Any,
+    ) -> Dict[str, Any]:
+        """Add a key to locale data, creating nested structure if needed.
+
+        Args:
+            data: Existing locale data.
+            key: Key in dot notation.
+            value: Value to set.
+
+        Returns:
+            Updated locale data.
+
+        """
+        parts = key.split(".")
+        current = data
+        for part in parts[:-1]:
+            if part not in current:
+                current[part] = {}
+            current = current[part]
+        current[parts[-1]] = value
+        return data
+
+    def get_value(self, data: Dict[str, Any], key: str) -> Optional[Any]:
+        """Get a value from locale data using dot notation.
+
+        Args:
+            data: Locale data dictionary.
+            key: Key in dot notation.
+
+        Returns:
+            The value if found, None otherwise.
+
+        """
+        parts = key.split(".")
+        current = data
+        for part in parts:
+            if isinstance(current, dict) and part in current:
+                current = current[part]
+            else:
+                return None
+        return current