i18n: add LANGUAGES, ACTIVE_LANGUAGES, respond_in_clause helper

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/services/i18n.py

@@ -0,0 +1,48 @@
+"""Language registry + prompt helpers for localized AI output.
+
+Two surfaces consume this module:
+- Per-user LLM call sites (portfolio analysis only at this stage) call
+  ``respond_in_clause(user.lang)`` and append the result to their
+  system prompt.
+- The settings dropdown + its PATCH endpoint consult ``ACTIVE_LANGUAGES``
+  to decide which options are selectable. The strategic-log and digest
+  translation fan-outs also consult it to decide which languages to
+  spend tokens on.
+
+Adding Spanish/French/German support later is a one-line constant
+change: extend ``ACTIVE_LANGUAGES`` to include the new code. No other
+code change is required — the rest of the system already treats them
+as first-class via ``LANGUAGES``.
+"""
+from __future__ import annotations
+
+
+# Display labels for every language the system knows about. ES/FR/DE
+# are kept here so labels still render in the dropdown (as disabled
+# options) without requiring code changes to enable them later.
+LANGUAGES: dict[str, str] = {
+    "en": "English",
+    "it": "Italian",
+    "es": "Spanish",
+    "fr": "French",
+    "de": "German",
+}
+
+
+# Languages users can actually select. Settings POST validates against
+# this; the strategic-log + digest translation fan-outs only consider
+# these.
+ACTIVE_LANGUAGES: set[str] = {"en", "it"}
+
+
+def respond_in_clause(lang: str | None) -> str:
+    """Suffix appended to per-user LLM system prompts.
+
+    Returns an empty string for ``en`` (no nudge needed), an unknown
+    code, or ``None``/empty input — those callers want the default
+    English path. Otherwise returns ``"\\n\\nRespond in <Language>."``
+    keyed off ``LANGUAGES``.
+    """
+    if not lang or lang == "en" or lang not in LANGUAGES:
+        return ""
+    return f"\n\nRespond in {LANGUAGES[lang]}."