Support number formatting from user settings

This change checks the current locale in the document to select
which locale should be used when formatting numbers.

app/assets/javascripts/lib/utils/unit_format/formatter_factory.js

@@ -20,8 +20,9 @@ function formatNumber(
     return '';
   }
 
+  const locale = document.documentElement.lang || undefined;
   const num = value * valueFactor;
-  const formatted = num.toLocaleString(undefined, {
+  const formatted = num.toLocaleString(locale, {
     minimumFractionDigits: fractionDigits,
     maximumFractionDigits: fractionDigits,
     style,

spec/frontend/lib/utils/unit_format/formatter_factory_spec.js

@@ -36,6 +36,27 @@ describe('unit_format/formatter_factory', () => {
       expect(formatNumber(10 ** 7, undefined, 9)).toBe('1.00e+7');
       expect(formatNumber(10 ** 7, undefined, 10)).toBe('10,000,000');
     });
+
+    describe('formats with a different locale', () => {
+      let originalLang;
+
+      beforeAll(() => {
+        originalLang = document.documentElement.lang;
+        document.documentElement.lang = 'es';
+      });
+
+      afterAll(() => {
+        document.documentElement.lang = originalLang;
+      });
+
+      it('formats a using the correct thousands separator', () => {
+        expect(formatNumber(1000000)).toBe('1.000.000');
+      });
+
+      it('formats a using the correct decimal separator', () => {
+        expect(formatNumber(12.345)).toBe('12,345');
+      });
+    });
   });
 
   describe('suffixFormatter', () => {