Format Localized App Messages | Dataford Interview Questions

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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.

Constraints

1 <= number of locales <= 10^3
1 <= number of keys across all locales <= 10^4
0 <= number of params <= 50
Locale strings may include a base language and region separated by -
Template placeholders use {placeholder} syntax

Function Signature

def format_message(translations, locale, key, params):