Add page_lang parameter to st.set_page_config by atharv-sys32 · Pull Request #16024 · streamlit/streamlit

atharv-sys32 · Jul 15, 2026

Describe your changes

Adds a page_lang parameter to st.set_page_config() that sets the lang attribute on the root <html> element. Previously this was hardcoded to "en", causing unwanted browser translation prompts for non-English apps and incorrect screen reader pronunciation.

GitHub Issue Link (if applicable)

Fixes #15726

Testing Plan

Changes follow the exact same pattern as the existing title / favicon handling. Proto field added, Python backend wires it, frontend sets it.

Note

Low Risk
Small, additive API and DOM attribute change with no auth or data-path impact; behavior matches existing page config patterns.

Overview
Adds page_lang to st.set_page_config() so apps can set the root <html> lang attribute instead of relying on the static en in the shell HTML.

The value flows through a new PageConfig.lang proto field; Python forwards non-empty tags on page_config_changed, and the frontend updates document.documentElement.lang when pageConfigChanged includes lang. Docs describe BCP 47 tags and the same additive/inherit behavior as other page config options (default en when never set).

^{Reviewed by Cursor Bugbot for commit 0e6b338. Bugbot is set up for automated code reviews on this repo. Configure here.}

Add a page_lang parameter to st.set_page_config that sets the lang attribute on the root <html> element via PageConfig proto. Previously the value was hardcoded to "en", which caused unwanted browser translation prompts for non-English apps. Changes: - PageConfig.proto: add string lang = 7 field - page_config.py: add page_lang parameter, wire to protobuf - App.tsx: set document.documentElement.lang when config arrives Fixes streamlit#15726

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snyk-io · Jul 15, 2026

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github-actions · Jul 15, 2026

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greptile-apps · Jul 15, 2026

Greptile Summary

This PR adds configurable page-language metadata to Streamlit apps. The main changes are:

A page_lang parameter for st.set_page_config().
A protobuf field carrying the language to the frontend.
An update to the root HTML lang attribute when a language is provided.

Confidence Score: 5/5

No additional blocking issue was found in the updated code.

The empty protobuf default no longer clears the root HTML language.
No separate production failure requiring an additional change was identified.

Important Files Changed

Filename	Overview
lib/streamlit/commands/page_config.py	Adds the public `page_lang` parameter and includes non-empty values in page configuration messages.
frontend/app/src/App.tsx	Applies the configured language to the root HTML element while preserving the existing value when omitted.
proto/streamlit/proto/PageConfig.proto	Adds the backward-compatible `lang` string field to `PageConfig`.

_{Reviews (3): Last reviewed commit: "Fix: use falsy check for lang proto3 def..." | Re-trigger Greptile}

greptile-apps · Jul 15, 2026

+    if page_lang is not None:
+        msg.page_config_changed.lang = page_lang


Invalid Language Tags Pass Through

Any non-None string, including "" or "not_a_language", is forwarded despite the documented BCP 47 requirement. The frontend then either ignores the empty value and leaves a stale language in place or assigns an invalid <html lang> value, so browsers and screen readers can continue using the wrong language.

graphite-app · Jul 15, 2026

+    if (lang) {
+      document.documentElement.lang = lang
+    }


The condition if (lang) will fail for empty strings since empty strings are falsy in JavaScript. However, the Python backend at page_config.py:309-310 checks if page_lang is not None, which means empty strings will be sent to the frontend but not applied. This creates an inconsistency.

Fix by checking explicitly for null/undefined:

if (lang !== undefined && lang !== null) { document.documentElement.lang = lang }

Or if empty strings should be rejected, add validation in the Python backend:

if page_lang is not None and page_lang != "": msg.page_config_changed.lang = page_lang

Suggested change

if (lang) {

document.documentElement.lang = lang

}

if (lang !== undefined && lang !== null) {

document.documentElement.lang = lang

}

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…ull check in frontend

cursor

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

^{Reviewed by Cursor Bugbot for commit f7e90f7. Configure here.}

protobuf3 defaults unset string fields to "", which is falsy in JS. Using if (lang) correctly skips the empty proto default so the original index.html lang="en" (or a previously set value) is not overwritten. Python backend already rejects empty strings.

Copilot AI review requested due to automatic review settings July 15, 2026 19:36

Copilot AI reviewed Jul 15, 2026

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graphite-app Bot reviewed Jul 15, 2026

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Fix: reject empty string for page_lang in backend and use defensive n…

f7e90f7

…ull check in frontend

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Add page_lang parameter to st.set_page_config#16024

Add page_lang parameter to st.set_page_config#16024
atharv-sys32 wants to merge 3 commits into
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atharv-sys32 commented Jul 15, 2026 •

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Confidence Score: 5/5

Important Files Changed

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		if page_lang is not None:
		msg.page_config_changed.lang = page_lang

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atharv-sys32 commented Jul 15, 2026 •

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