feat: add Russian localization and independent STT languages

src/agent/teacher-agent.ts

@@ -1,3 +1,4 @@
+import { learningLanguageInstruction, type Language } from "../core/i18n.ts";
 import { Agent } from "@earendil-works/pi-agent-core";
 import type { AgentEvent, AgentMessage, AgentTool } from "@earendil-works/pi-agent-core";
 import { invalidInputResult, questionResult } from "./tool-execute.ts";
@@ -97,6 +98,7 @@ export type StepUsageCollector = (usage: Usage) => void;
 
 /** Per-request construction options (the handler wires all of these). */
 export interface AgentBuildOptions {
+	language?: Language;
 	/** Model + stream function — tests inject a scripted faux engine. */
 	engine?: PiEngine;
 	callPurpose?: { background: boolean };
@@ -175,9 +177,10 @@ function baseAgentOptions(kind: AgentKind, opts: AgentBuildOptions, tools: Agent
 	// ("practice" is not a ModelRole) honest about that.
 	const engine = opts.engine ?? piEngineFor(kind as "prober" | "teacher");
 	const stopper = makeQuestionStopper();
-	const systemPrompt = opts.promptSwitches
+	const basePrompt = opts.promptSwitches
 		? composeInstructions(kind, opts.promptSwitches)
 		: composedInstructions[kind]!;
+	const systemPrompt = `${basePrompt}\n\n${learningLanguageInstruction(opts.language ?? "en")}`;
 	// Debug recorder (owner, 2026-10-04): raw provider events when
 	// LH_PROVIDER_EVENTS=1 and the caller handed a chatId; undefined is
 	// the normal no-observer path (tests, env-off).