feat(surveys): re-translate displayed survey on language change

[bs1180]

💡 Motivation and Context

Once a translated survey was displayed, later changes to the user's language (e.g. via setPersonProperties / identify with a language property) had no effect — the survey kept the language it was first rendered with. The translation was resolved only at display time.

This makes a survey already on screen follow the user's language: when the person properties used for flags change and a matching translation exists, the survey re-translates in place, keeping the current question and any progress.

Closes #66998

💚 How did you test it?

Added unit tests (run on the iOS simulator, where survey code compiles):

• Changing the language person property while a survey is active pushes a re-translated survey to the delegate and updates the tracked survey language.
• Re-resolving to the same language is a no-op (no churn of $survey_language or re-renders).
• No active survey ⇒ no update pushed.
• SurveyDisplayController.updateSurvey preserves the current question index and completion state, and ignores updates for a different / absent survey.

I was unable to run make test / the iOS build in this environment (no Xcode available), so CI should be relied on for the full build + simulator test run.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Implemented from a Slack thread request, logged as #66998. Built with the PostHog Slack app (Claude Code). Key decisions:

• Trigger: chose an internal NotificationCenter signal posted from PostHogRemoteConfig.setPersonPropertiesForFlags (the single chokepoint all person-property updates flow through) rather than sniffing the captured event stream — the $set/$identify ordering relative to the flag-property update is inconsistent, so the notification is the order-correct, robust hook.
• UI update: added an optional updateSurvey(_:) to the delegate and made SurveySheet observe the display controller (instead of the snapshot passed by .sheet(item:)), so an in-place content swap with the same survey id renders live while preserving SwiftUI @State (current question, typed answers).
• Guardrails: only commit + render when the matched language actually changes; the whole path is a no-op when no survey is displayed.

One known limitation worth a reviewer's eye: for choice questions, selection is keyed by the option's string value, so a mid-survey language switch can drop an in-progress selection on those question types (labels change). Text/rating answers are preserved.

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Created with PostHog from a Slack thread

[greptile-apps]

Comments Outside Diff (1)

1. PostHog/Surveys/PostHogSurveysDefaultDelegate.swift, line 120-123 (link)

   Translation update dropped when survey has a display delay

   When updateSurvey is called while a survey is still within its surveyPopupDelaySeconds window, it correctly updates self.pendingSurvey to the re-translated copy. However, the DispatchWorkItem closure captures the original survey local variable at the time it is created, so when the delay fires it always calls displayController.showSurvey(survey) with the pre-update content. The work item should use self.pendingSurvey — the most-recent version — at the point it executes.

_{Reviews (1): Last reviewed commit: "feat(surveys): re-translate displayed su..." | Re-trigger Greptile}

[github-actions]

posthog-ios Compliance Report

Date: 2026-07-06 12:23:58 UTC
Duration: 211112ms

✅ All Tests Passed!

45/45 tests passed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	2874ms
Format Validation.Event Has Uuid	✅	2787ms
Format Validation.Event Has Lib Properties	✅	2700ms
Format Validation.Distinct Id Is String	✅	2752ms
Format Validation.Token Is Present	✅	2749ms
Format Validation.Custom Properties Preserved	✅	340ms
Format Validation.Event Has Timestamp	✅	2666ms
Retry Behavior.Retries On 503	✅	11347ms
Retry Behavior.Does Not Retry On 400	✅	4729ms
Retry Behavior.Does Not Retry On 401	✅	2318ms
Retry Behavior.Respects Retry After Header	✅	7757ms
Retry Behavior.Implements Backoff	✅	21357ms
Retry Behavior.Retries On 500	✅	9171ms
Retry Behavior.Retries On 502	✅	9183ms
Retry Behavior.Retries On 504	✅	9286ms
Retry Behavior.Max Retries Respected	✅	21742ms
Deduplication.Generates Unique Uuids	✅	482ms
Deduplication.Preserves Uuid On Retry	✅	9274ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10258ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	7704ms
Deduplication.No Duplicate Events In Batch	✅	2880ms
Deduplication.Different Events Have Different Uuids	✅	2832ms
Compression.Sends Gzip When Enabled	✅	2655ms
Batch Format.Uses Proper Batch Structure	✅	2797ms
Batch Format.Flush With No Events Sends Nothing	✅	224ms
Batch Format.Multiple Events Batched Together	✅	2823ms
Error Handling.Does Not Retry On 403	✅	2383ms
Error Handling.Does Not Retry On 413	✅	2348ms
Error Handling.Retries On 408	✅	5237ms

Feature_Flags Tests

✅ 16/16 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	2752ms
Request Payload.Flags Request Uses V2 Query Param	✅	2788ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	2665ms
Request Payload.Flags Request Omits Authorization Header	✅	2653ms
Request Payload.Token In Flags Body Matches Init	✅	2844ms
Request Payload.Groups Round Trip	✅	2681ms
Request Payload.Groups Default To Empty Object	✅	2764ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	✅	2703ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	2730ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	2805ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	2777ms
Request Lifecycle.No Flags Request On Init Alone	✅	114ms
Request Lifecycle.No Flags Request On Normal Capture	✅	2931ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	5625ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	2920ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	3049ms

[turnipdabeets]

@lucasheriques I know you worked a bit on this, mind giving this a review?

[greptile-apps]

_{Reviews (2): Last reviewed commit: "fix(surveys): apply latest translation w..." | Re-trigger Greptile}

[turnipdabeets]

Thanks @bs1180 instead of a CHANGELOG we actually need a changes, please see RELEASING.md

[ioannisj]

@bs1180 friendly ping on this one

[bs1180]

Hey @turnipdabeets and @ioannisj, sorry for the delay. This PR is the product of letting PostHog AI loose on a customer complaint :D I'm not an IOS dev so not sure if this implementation makes sense and I've done literally zero QA. Is that OK with you? Won't be offended this just gets rejected.

[ioannisj]

@bs1180 thank you, I can do the QA part and see also how this will affect Flutter surveys as well. we'll probably need to land a similar feature for posthog-android as well I imagine? To get some more context though, do you have the original link to the customer complaint? I personally don't think that on-the-fly language changes are the norm so I'm not completely sure we should be supporting this

[ioannisj]

Would like to know @turnipdabeets thoughts as well on this, maybe it's just me :)

[bs1180]

Yup totally understood - PostHog/posthog#66998 is the original feature request, and this is the customer Slack thread

[turnipdabeets]

Would like to know @turnipdabeets thoughts as well on this, maybe it's just me :)

I think we need on-device QA, if you want to take that @ioannisj. Maybe we need to check out the sheet dismissal animation too. Not sure if we need @PostHog/team-surveys on par before making this change? And it makes sense to do this for the other mobile if it's needed, android flutter and RN.