diff --git a/src/gdiffer/language_detector.py b/src/gdiffer/language_detector.py
new file mode 100644
index 0000000..344b600
--- /dev/null
+++ b/src/gdiffer/language_detector.py
@@ -0,0 +1,75 @@
+"""Language detection for code files."""
+
+from typing import Optional
+
+
+class LanguageDetector:
+    EXTENSION_MAP = {
+        'py': 'python', 'pyw': 'python', 'pyx': 'python',
+        'js': 'javascript', 'mjs': 'javascript', 'cjs': 'javascript', 'jsx': 'javascript',
+        'ts': 'typescript', 'tsx': 'typescript', 'mts': 'typescript', 'cts': 'typescript',
+        'java': 'java', 'kt': 'kotlin', 'kts': 'kotlin',
+        'go': 'go', 'rs': 'rust', 'c': 'c', 'h': 'c',
+        'cpp': 'cpp', 'cc': 'cpp', 'cxx': 'cpp', 'hpp': 'cpp', 'hxx': 'cpp',
+        'cs': 'csharp', 'rb': 'ruby', 'erb': 'ruby', 'php': 'php',
+        'swift': 'swift', 'm': 'objective-c', 'mm': 'objective-c',
+        'scala': 'scala', 'sc': 'scala', 'jl': 'julia',
+        'r': 'r', 'R': 'r', 'lua': 'lua',
+        'pl': 'perl', 'pm': 'perl', 'sql': 'sql',
+        'sh': 'bash', 'bash': 'bash', 'zsh': 'bash', 'fish': 'bash',
+        'yaml': 'yaml', 'yml': 'yaml', 'json': 'json',
+        'xml': 'xml', 'html': 'html', 'htm': 'html',
+        'css': 'css', 'scss': 'scss', 'sass': 'sass', 'less': 'less',
+        'md': 'markdown', 'markdown': 'markdown',
+        'txt': 'text', 'dockerfile': 'dockerfile', 'Dockerfile': 'dockerfile',
+    }
+
+    CONTENT_PATTERNS = {
+        'python': [r'^import\s+\w+', r'^from\s+\w+\s+import', r'^def\s+\w+\s*\(', r'^class\s+\w+'],
+        'javascript': [r'^const\s+\w+', r'^let\s+\w+', r'^var\s+\w+', r'^function\s+\w+', r'=>\s*\{'],
+        'typescript': [r'^interface\s+\w+', r'^type\s+\w+', r':\s*(string|number|boolean)'],
+        'java': [r'^package\s+[\w.]+;', r'^import\s+[\w.]+;', r'^public\s+class\s+\w+'],
+        'go': [r'^package\s+\w+', r'^import\s+\(', r'func\s+\w+'],
+        'rust': [r'^fn\s+\w+', r'^impl\s+\w+', r'^struct\s+\w+', r'^enum\s+\w+'],
+        'c': [r'#include\s*<', r'#include\s*"', r'int\s+main\s*\('],
+        'cpp': [r'#include\s*<', r'#include\s*"', r'class\s+\w+', r'std::\w+'],
+        'ruby': [r'^require\s+', r'^class\s+\w+', r'^module\s+\w+', r'def\s+\w+'],
+        'php': [r'<\?php', r'\$\w+\s*=', r'function\s+\w+', r'class\s+\w+'],
+    }
+
+    def detect_from_filename(self, filename: str) -> Optional[str]:
+        if '.' not in filename:
+            return None
+        ext = filename.rsplit('.', 1)[-1].lower()
+        return self.EXTENSION_MAP.get(ext)
+
+    def detect_from_content(self, content: str) -> Optional[str]:
+        first_lines = '\n'.join(content.splitlines()[:50])
+        scores = {}
+        for lang, patterns in self.CONTENT_PATTERNS.items():
+            import re
+            score = sum(len(re.findall(p, first_lines, re.MULTILINE)) for p in patterns)
+            if score > 0:
+                scores[lang] = score
+        return max(scores, key=scores.get) if scores else None
+
+    def detect(self, filename: str, content: str = "") -> str:
+        ext_lang = self.detect_from_filename(filename)
+        if ext_lang and ext_lang not in ['text', 'markdown', 'json', 'yaml', 'xml', 'html', 'css', 'dockerfile']:
+            return ext_lang
+        if content:
+            content_lang = self.detect_from_content(content)
+            if content_lang:
+                return content_lang
+        return ext_lang or "text"
+
+    def get_supported_languages(self) -> list[str]:
+        return sorted(set(self.EXTENSION_MAP.values()))
+
+    def is_language_supported(self, language: str) -> bool:
+        return language in self.get_supported_languages()
+
+
+def detect_language(filename: str, content: str = "") -> str:
+    detector = LanguageDetector()
+    return detector.detect(filename, content)