test(i18n): guard that every report agent applies the output language

The output-language instruction is applied across all report-producing agents
(analysts, researchers, risk debators, research manager, trader, portfolio
manager), but nothing enforced it, so agents had silently dropped it before. Add
a parametrized guard asserting each report agent calls get_language_instruction()
so a non-English run stays fully localized and the regression can't recur.

tests/test_i18n_coverage.py

@@ -0,0 +1,59 @@
+"""Every report-producing agent must apply the configured output language
+(#740/#801).
+
+A non-English run should produce a fully localized report, not a mix of
+languages. The bug originally happened because several agents silently omitted
+the instruction (fixed in 6b384f7); this test codifies the invariant so a future
+refactor can't quietly drop it again.
+"""
+from pathlib import Path
+
+import pytest
+
+from tradingagents.agents.utils.agent_utils import get_language_instruction
+
+_AGENTS_DIR = Path(__file__).resolve().parents[1] / "tradingagents" / "agents"
+
+# Every node whose text reaches the saved report. If you add a report-producing
+# agent, add it here — and make it call get_language_instruction().
+REPORT_AGENTS = [
+    "analysts/market_analyst.py",
+    "analysts/news_analyst.py",
+    "analysts/fundamentals_analyst.py",
+    "analysts/sentiment_analyst.py",
+    "researchers/bull_researcher.py",
+    "researchers/bear_researcher.py",
+    "managers/research_manager.py",
+    "managers/portfolio_manager.py",
+    "risk_mgmt/aggressive_debator.py",
+    "risk_mgmt/conservative_debator.py",
+    "risk_mgmt/neutral_debator.py",
+    "trader/trader.py",
+]
+
+
+@pytest.mark.unit
+class TestLanguageInstruction:
+    def test_english_adds_no_tokens(self, monkeypatch):
+        from tradingagents.dataflows.config import set_config
+        set_config({"output_language": "English"})
+        assert get_language_instruction() == ""
+
+    def test_non_english_emits_directive(self):
+        from tradingagents.dataflows.config import set_config
+        set_config({"output_language": "中文"})
+        out = get_language_instruction()
+        assert "中文" in out
+        assert "entire response" in out
+
+
+@pytest.mark.unit
+@pytest.mark.parametrize("rel", REPORT_AGENTS)
+def test_report_agent_applies_language_instruction(rel):
+    path = _AGENTS_DIR / rel
+    assert path.exists(), f"missing agent module: {rel}"
+    src = path.read_text(encoding="utf-8")
+    assert "get_language_instruction()" in src, (
+        f"{rel} does not apply get_language_instruction(); its output would "
+        f"ignore the configured output_language (#740/#801)."
+    )