Problem

A global mobile app stores message templates for different locales. Implement a formatter that selects the best available localized template and replaces placeholders with provided values.

Write a function format_message(translations, locale, key, params) that returns the formatted string for a message key.

Formal Specification

• Input:
  • translations: a dictionary where each locale maps to another dictionary of message keys to template strings
  • locale: a locale string such as "en-US" or "fr-CA"
  • key: the message key to render
  • params: a dictionary of placeholder names to replacement values
• Output:
  • A formatted string using this fallback order:
    1. exact locale match
    2. base language match (for example, en for en-US)
    3. default locale en
    4. the raw key if no template exists

Placeholders use {name} syntax. Replace only placeholders present in params; leave unknown placeholders unchanged.

Examples

Example 1

• Input: translations = {"en": {"greet": "Hello, {name}!"}, "fr": {"greet": "Bonjour, {name}!"}}, locale = "fr-CA", key = "greet", params = {"name": "Ava"}
• Output: "Bonjour, Ava!"
• Explanation: fr-CA is missing, so the function falls back to base language fr.

Example 2

• Input: translations = {"en": {"items": "{count} items"}}, locale = "de-DE", key = "items", params = {"count": 3}
• Output: "3 items"
• Explanation: Neither de-DE nor de exists, so the function falls back to en.

Constraints

• 1 <= number of locales <= 10^3
• 1 <= number of keys across all locales <= 10^4
• 0 <= number of params <= 50
• Locale, key, and placeholder names contain printable ASCII characters