feat: Add multi-language support for CrewAI prompts

- Add language parameter to I18N class to support multiple languages
- Implement fallback to English for unsupported languages
- Add Spanish (es.json) translation file as example
- Update Agent and Crew classes to accept language parameter
- Add comprehensive tests for multi-language support

Fixes #3780

Co-Authored-By: João <joao@crewai.com>

lib/crewai/tests/test_agent_language.py

@@ -0,0 +1,116 @@
+import pytest
+from crewai.agent import Agent
+from crewai.crew import Crew
+from crewai.task import Task
+
+
+def test_agent_default_language():
+    agent = Agent(
+        role="Test Agent",
+        goal="Test goal",
+        backstory="Test backstory"
+    )
+    assert agent.i18n.language == "en"
+
+
+def test_agent_with_spanish_language():
+    agent = Agent(
+        role="Agente de Prueba",
+        goal="Objetivo de prueba",
+        backstory="Historia de fondo de prueba",
+        language="es"
+    )
+    assert agent.i18n.language == "es"
+    assert "Eres {role}" in agent.i18n.slice("role_playing")
+
+
+def test_agent_with_english_language_explicit():
+    agent = Agent(
+        role="Test Agent",
+        goal="Test goal",
+        backstory="Test backstory",
+        language="en"
+    )
+    assert agent.i18n.language == "en"
+    assert "You are {role}" in agent.i18n.slice("role_playing")
+
+
+def test_crew_default_language():
+    agent = Agent(
+        role="Test Agent",
+        goal="Test goal",
+        backstory="Test backstory"
+    )
+    task = Task(
+        description="Test task",
+        expected_output="Test output",
+        agent=agent
+    )
+    crew = Crew(agents=[agent], tasks=[task])
+    assert crew.language is None
+
+
+def test_crew_with_spanish_language():
+    agent = Agent(
+        role="Agente de Prueba",
+        goal="Objetivo de prueba",
+        backstory="Historia de fondo de prueba"
+    )
+    task = Task(
+        description="Tarea de prueba",
+        expected_output="Salida de prueba",
+        agent=agent
+    )
+    crew = Crew(agents=[agent], tasks=[task], language="es")
+    assert crew.language == "es"
+
+
+def test_crew_language_propagates_to_agents():
+    agent1 = Agent(
+        role="Agent 1",
+        goal="Goal 1",
+        backstory="Backstory 1"
+    )
+    agent2 = Agent(
+        role="Agent 2",
+        goal="Goal 2",
+        backstory="Backstory 2"
+    )
+    task = Task(
+        description="Test task",
+        expected_output="Test output",
+        agent=agent1
+    )
+    crew = Crew(agents=[agent1, agent2], tasks=[task], language="es")
+    
+    assert crew.language == "es"
+
+
+def test_agent_language_overrides_default():
+    agent_en = Agent(
+        role="English Agent",
+        goal="English goal",
+        backstory="English backstory",
+        language="en"
+    )
+    agent_es = Agent(
+        role="Spanish Agent",
+        goal="Spanish goal",
+        backstory="Spanish backstory",
+        language="es"
+    )
+    
+    assert agent_en.i18n.language == "en"
+    assert agent_es.i18n.language == "es"
+    assert "You are {role}" in agent_en.i18n.slice("role_playing")
+    assert "Eres {role}" in agent_es.i18n.slice("role_playing")
+
+
+def test_agent_without_language_uses_default():
+    agent = Agent(
+        role="Test Agent",
+        goal="Test goal",
+        backstory="Test backstory"
+    )
+    assert agent.language is None
+    assert agent.i18n.language == "en"