Add Get User's Preferred Language From Browser as a JavaScript TIL

javascript/get-users-preferred-language-from-browser.md

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+# Get User's Preferred Language From Browser
+
+A great way to determine a user's preferred language if you aren't able to ask
+them directly is to look at the language setting for their browser's UI.
+
+We can get this from the instance of
+[`Navigator`](https://developer.mozilla.org/en-US/docs/Web/API/Navigator) in the
+user's JavaScript runtime within the browser.
+
+My browser's UI is set to US English, which I can verify like so:
+
+```javascript
+> navigator.language
+'en-US'
+```
+
+This is useful for all sorts of things like [formatting dates for
+display](basic-date-formatting-without-a-library.md):
+
+```javascript
+> const now = new Date();
+> Intl.DateTimeFormat(navigator.language).format(now)
+'5/14/2026'
+```
+
+Or for [formatting other kinds of units for
+display](formatting-values-with-units-for-display.md):
+
+```javascript
+> const milesFormat =
+    Intl.NumberFormat(navigator.language, { style: "unit", unit: "mile" });
+> milesFormat.format(1500)
+"1,500 mi"
+```