feat: Add multi-language support for CrewAI prompts (Fixes #3780)

[devin-ai-integration]

feat: Add multi-language support for CrewAI prompts (Fixes #3780)

Summary

This PR adds multi-language support to CrewAI, allowing users to specify a language for agent and crew prompts. Previously, all prompts were hardcoded in English. Now users can:

1. Set a language parameter on Agent or Crew to use translations in their preferred language
2. CrewAI falls back to English if a requested language translation is not available
3. A complete Spanish translation (es.json) is included as an example

Key Changes:

• Extended I18N class with language parameter and fallback logic
• Added language field to BaseAgent and Crew classes
• Created Spanish translation file with all prompts, errors, tools, and reasoning messages
• Added comprehensive test coverage for multi-language functionality

Review & Testing Checklist for Human

⚠️ IMPORTANT: The automated tests could not be run due to environment setup issues during development. Manual validation was performed but pytest execution needs verification.

• Spanish Translation Accuracy (HIGH PRIORITY): Review es.json for translation quality. The translations were AI-generated and should be validated by a native Spanish speaker for accuracy, natural phrasing, and appropriate technical terminology
• Test Execution: Run uv run pytest lib/crewai/tests/utilities/test_i18n_multilanguage.py lib/crewai/tests/test_agent_language.py -vv to verify all tests pass in a clean environment
• Language Priority Logic: Test behavior when both crew and agent language parameters are set - verify the crew language properly propagates to agents during kickoff()
• Fallback Behavior: Test with an unsupported language code (e.g., language="fr") to confirm it falls back to English gracefully
• End-to-End Integration: Run a simple crew with language="es" and verify that:
  • Agent prompts are in Spanish
  • Error messages are in Spanish
  • Tool descriptions are in Spanish
  • The crew executes successfully

Test Plan

# Quick smoke test
from crewai import Agent, Crew, Task

agent = Agent(
    role="Investigador",
    goal="Investigar temas",
    backstory="Eres un investigador experto",
    language="es"
)

task = Task(
    description="Investigar Python",
    expected_output="Un informe sobre Python",
    agent=agent
)

crew = Crew(agents=[agent], tasks=[task], language="es")
result = crew.kickoff()

Notes

• The implementation follows the existing pattern where prompt_file can override language-based translations
• Fallback to English is automatic if a translation file doesn't exist
• The Spanish translation file is comprehensive (~310 lines) and includes all prompt categories: slices, errors, tools, reasoning, and hierarchical_manager_agent
• This is a backward-compatible change - all existing code continues to work with English as the default

Session Info:

• Requested by: João (joao@crewai.com) in Slack #joao-test
• Devin Session: https://app.devin.ai/sessions/1f7d7b2e75e84da088b068bd794835ed

[devin-ai-integration]

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