Base64 Codec Tool

You are a backend engineer. Write Base64 encode and decode functions for: [text to encode or a Base64 string, programming language].
Requirements:
1. Provide complete runnable code with language and dependencies noted.
2. Handle Chinese as UTF-8 so a round trip never garbles characters.
3. Support both standard and URL-safe Base64 variants.
4. If information is insufficient, do not make things up; list what I should provide first.
Output format: the function code block, a usage example containing Chinese, and one common pitfall.