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Set Chrome’s language preference before creating the driver. Selenium passes Chromium’s intl.accept_languages preference through ChromeOptions, while --headless=new starts current headless Chrome:
from selenium import webdriver
from selenium.webdriver.chrome.options import Options
options = Options()
options.add_argument("--headless=new")
options.add_experimental_option(
"prefs",
{"intl.accept_languages": "fr-FR,fr"},
)
driver = webdriver.Chrome(options=options)
try:
driver.get("https://example.com/")
finally:
driver.quit()Replace fr-FR,fr with an ordered, comma-separated list such as en-US,en or de-DE,de. The preference affects browser-generated requests, but it does not force an application to display a particular translation.
What intl.accept_languages controls
intl.accept_languages is Chromium’s preference for the language list used in the Accept-Language HTTP request header. The first value is preferred, followed by fallbacks. Selenium’s Python Options object sends that preference to Chrome when the session starts.
For example, fr-FR,fr asks for French as used in France, then generic French. A server may respond in French, but it can also use cookies, a locale in the URL, an account setting, geolocation, or its own negotiation rules instead.
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Install the dependencies
Install Selenium in the Python environment used by your test runner:
python -m pip install -U seleniumYou also need a compatible Chrome or Chromium installation. Modern Selenium can manage the driver in many environments; in pinned CI images, record the Chrome, Chromium and Selenium versions together so a later browser update does not silently change locale behavior.
Choose the language list and start a fresh session
from selenium import webdriver
from selenium.webdriver.chrome.options import Options
LANGUAGES = "de-DE,de"
options = Options()
options.add_argument("--headless=new")
options.add_experimental_option(
"prefs",
{"intl.accept_languages": LANGUAGES},
)
# A new driver is important: preferences are read when Chrome starts.
driver = webdriver.Chrome(options=options)
try:
driver.get("https://example.com/")
print(driver.title)
finally:
driver.quit()Use a new WebDriver instance after changing the preference. Reusing an existing session will not reliably recreate the renderer processes with the new setting.
Common language-list patterns
| Value | Typical intent |
|---|---|
en-US,en |
US English, then generic English |
de-DE,de |
German as used in Germany, then generic German |
fr-FR,fr |
French as used in France, then generic French |
ja-JP,ja |
Japanese as used in Japan, then generic Japanese |
es-ES,es |
Spanish as used in Spain, then generic Spanish |
Keep the order meaningful: Chrome treats the list as preference ordered. Do not add spaces unless your test intentionally models a header containing them; a compact comma-separated value is easiest to audit.
How to verify the actual language negotiation
Checking JavaScript alone is insufficient. A page can expose one language while the request header has been normalized, reduced, ignored, or overridden by application state.
Inspect browser-visible values as a diagnostic
language = driver.execute_script("return navigator.language")
languages = driver.execute_script("return navigator.languages")
print("navigator.language:", language)
print("navigator.languages:", languages)These values help identify the browser’s effective language settings, but they are not proof of the exact wire header. Chrome’s Accept-Language reduction can affect both the HTTP header and the navigator.languages getter, and the behavior depends on Chrome version and rollout.
Verify at the server or network layer
Use a request-inspection endpoint you control, or a controlled test server that records incoming headers. Assert the observed Accept-Language value there. This catches cases where an application redirects, applies a cookie, or ignores negotiation.
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For deterministic tests, clear or isolate cookies, avoid a URL that embeds a locale unless that is what you are testing, and start a new driver for each language scenario. A response body, redirect location, or server-side locale marker is usually a stronger assertion than a browser property.
Preference versus a DevTools request override
There are two different testing goals. Configure the preference when you want to model a user’s Chrome language list. Use a Chrome DevTools Protocol (CDP) network override only when you need narrowly scoped request instrumentation.
| Approach | Scope | JavaScript visibility | Persistence | Best use |
|---|---|---|---|---|
intl.accept_languages |
Broadly applies to browser-generated requests | Normally reflected in navigator.language and navigator.languages, subject to reduction |
Chrome profile preference for the session | Localized navigation and server content negotiation |
| CDP network override | Request-level instrumentation | Does not necessarily model the browser’s full language preference | Override-specific and version-sensitive | A targeted header experiment |
Selenium exposes Chromium’s CDP commands, but the exact command and behavior must match the Chrome and Selenium versions in your suite. Treat a CDP header override as a request-level technique, not a replacement for setting the browser preference when testing locale behavior end to end.
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Chrome 112 unified headless and headful Chrome: headless now uses the same implementation while creating platform windows that are not displayed. Current Selenium examples therefore use --headless=new. Chrome still documents the --headless flag, but avoid assumptions based on the obsolete “old headless” implementation.
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Why the expected language may not appear
The site chooses another locale
Servers can prioritize a cookie, URL parameter, account profile, geolocation, or an application default over Accept-Language. Remove those influences when testing HTTP negotiation, or include them deliberately when testing the application’s real precedence rules.
Chrome normalizes or reduces the header
Chromium composes the effective preference from selected and forced language lists and may normalize values. Chrome’s Accept-Language reduction can change what appears in the request and in JavaScript. Pin browser versions in CI and assert the server-observed result instead of assuming the configured string is transmitted byte for byte.
The preference was applied too late
Preferences are consumed during startup. Create a new driver after changing Options; do not expect an already-running session to adopt the new list.
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The wrong preference key or value was used
The key is exactly intl.accept_languages. It takes a string such as fr-FR,fr, not a Python list and not an HTTP-header dictionary. Keep the preference inside add_experimental_option("prefs", ...).
The browser and driver are incompatible
A startup failure, session creation error, or inconsistent behavior can come from incompatible Chrome and driver versions or a stale CI image. Update or pin the pair, print their versions in CI logs, and rerun the same locale test in a clean session.
Testing several locales safely
Parameterize the language list, but create and destroy a driver for every case. This prevents cookies, local storage, service workers, and renderer state from leaking between locales:
from selenium import webdriver
from selenium.webdriver.chrome.options import Options
cases = [
("en-US,en", "English"),
("fr-FR,fr", "French"),
("de-DE,de", "German"),
]
for language_value, label in cases:
options = Options()
options.add_argument("--headless=new")
options.add_experimental_option(
"prefs",
{"intl.accept_languages": language_value},
)
driver = webdriver.Chrome(options=options)
try:
driver.get("https://example.com/")
observed = driver.execute_script("return navigator.languages")
print(label, language_value, observed)
finally:
driver.quit()For a real localization assertion, replace the diagnostic print with a server-side assertion or a check for a locale marker that your application intentionally exposes.
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- Startup cost: a fresh Chrome process per locale is slower than reusing one browser, but it gives isolation and ensures the preference is loaded.
- Parallel runs: use separate temporary profiles and independent driver instances. Shared profiles can carry cookies and language state across tests.
- Network variability: redirects, cache state and delayed locale requests can make a page appear inconsistent. Wait for a meaningful application condition, not only document readiness.
- Evidence: log Chrome version, Selenium version, configured language list, final URL and server-observed header for failures.
- Scope: browser preferences affect browser-generated requests. They do not rewrite arbitrary requests made by application code or guarantee that every third-party service honors the value.
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Frequently Asked Questions
Does setting intl.accept_languages translate the page automatically?
No. It communicates a preferred language to the browser and usually to servers through content negotiation. The application still decides whether and how to translate its response.
Can I change the language preference after Chrome has started?
Start a new WebDriver session with a new Options object. Preferences are read during startup, so changing Python data in an existing session is not a reliable way to change renderer behavior.
Is navigator.language enough to test Accept-Language?
No. Chrome can reduce or normalize language values, and a site can ignore them. Verify the header at a server or network layer when the wire value matters.
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