Gate a locale behind a feature flag

You've launched fr:prod — drafted, reviewed, published. But a freshly machine-translated locale carries risk you can't fully see in review: a string that overflows a button, a register that reads wrong to native speakers, an edge case your reviewer missed. You don't want to discover it from all of France at once.

Treat the locale like any other feature: roll it out behind a feature flag before going 100%.

The idea: the locale exists and is published, but whether a French-locale user gets French is decided by a gate. Ramp the gate, watch for breakage, then open it to everyone.

1. Create the gate

shipeasy flags create french-locale --rollout 0

--rollout 0 ships it dark — the locale is published but nobody is routed to it yet.

2. Choose the locale through the gate

Resolve the user's effective locale by combining their browser/account preference with the gate:

import { flags } from "@shipeasy/sdk/server";

function effectiveLocale(user: { id: string; pref: string }) {
  if (user.pref === "fr" && flags.get("french-locale", { user_id: user.id })) {
    return "fr";
  }
  return "en"; // not yet eligible → safe fallback to the source locale
}

The SDK serves labels for whatever locale you ask it to render. A user who prefers French but isn't in the rollout simply sees English — no missing strings, no broken page.

3. Ramp it

shipeasy flags rollout french-locale 10   # a slice of French-preference users
shipeasy flags rollout french-locale 50   # half, once it looks clean
shipeasy flags rollout french-locale 100  # everyone — then retire the gate

Each step is sticky per user, so a French user who got French keeps French as you ramp — no flicker between languages.

Rollout & measurement plan

• Start with French-preference users only. The gate is layered under the preference check, so the rollout percentage applies to the population that would actually see French — your sample is the people who matter.
• Watch support + layout. A spike in French-locale bug reports (or error reports from French routes) is your signal to pause the ramp and fix a string.
• Retire the gate at 100%. Once French is fully live and stable, drop the gate from the locale-resolution code so French becomes unconditional for fr users.
• Reuse the pattern per locale. Each new language gets its own gate; you're never ramping two unproven locales on the same switch.