Add UseScaleWord format to auto-select scale based on locale

src/Humanizer/MetricNumeralExtensions.cs

@@ -494,6 +494,16 @@
             {
                 return UnitPrefixes[symbol].LongScaleWord;
             }
+
+            if (formatValue.HasFlag(MetricNumeralFormats.UseScaleWord))
+            {
+                var culture = CultureInfo.CurrentUICulture;
+                var isLongScale = LongScaleCultures.Contains(culture.Name)
+                               || LongScaleCultures.Contains(culture.TwoLetterISOLanguageName);
+                return isLongScale
+                    ? UnitPrefixes[symbol].LongScaleWord
+                    : UnitPrefixes[symbol].ShortScaleWord;
+            }
         }
 
         return symbol.ToString();
@@ -536,4 +546,38 @@
         public string ShortScaleWord { get; } = shortScaleWord;
         public readonly string LongScaleWord => longScaleWord ?? ShortScaleWord;
     }
-}
+
+
+    /// <summary>
+    /// A set of culture codes that use the <a href="https://en.wikipedia.org/wiki/Long_and_short_scales">long scale</a> system.
+    /// In the long scale, <c>1E9</c> is a <c>milliard</c> and <c>1E12</c> is a <c>billion</c>.
+    /// Cultures not in this set are assumed to use the short scale, where <c>1E9</c> is a <c>billion</c>.
+    /// Used by <see cref="MetricNumeralFormats.UseScaleWord"/> to automatically select the correct scale word
+    /// based on <see cref="System.Globalization.CultureInfo.CurrentUICulture"/>.
+    /// </summary>
+    static readonly HashSet<string> LongScaleCultures =
+    [
+        "de", "de-DE", "de-AT", "de-CH",  // German
+        "fr", "fr-FR", "fr-BE", "fr-CH",  // French
+        "it", "it-IT", "it-CH",            // Italian
+        "es", "es-ES",                     // Spanish
+        "pt", "pt-PT",                     // Portuguese (NOT pt-BR - Brazil uses short scale)
+        "nl", "nl-NL", "nl-BE",            // Dutch
+        "ru", "ru-RU",                     // Russian
+        "pl", "pl-PL",                     // Polish
+        "tr", "tr-TR",                     // Turkish
+        "cs", "cs-CZ",                     // Czech (miliarda for 1E9)
+        "sk", "sk-SK",                     // Slovak
+        "hr", "hr-HR",                     // Croatian
+        "ro", "ro-RO",                     // Romanian
+        "sl", "sl-SI",                     // Slovenian
+        "uk", "uk-UA",                     // Ukrainian
+        "he", "he-IL",                     // Hebrew (מיליארד for 1E9)
+        "ar", "ar-SA",                     // Arabic
+        "hu", "hu-HU",                     // Hungarian
+        "fi", "fi-FI",                     // Finnish
+        "nb", "nb-NO",                     // Norwegian
+        "sv", "sv-SE",                     // Swedish
+        "da", "da-DK",                     // Danish
+    ];
+}

src/Humanizer/MetricNumeralFormats.cs

@@ -24,5 +24,12 @@ public enum MetricNumeralFormats
     /// <summary>
     /// Include a space after the numeral.
     /// </summary>
-    WithSpace = 8
+    WithSpace = 8,
+
+    /// <summary>
+    /// Automatically select between <a href="https://en.wikipedia.org/wiki/Long_and_short_scales">long and short scale words</a>
+    /// based on <see cref="System.Globalization.CultureInfo.CurrentUICulture"/>.
+    /// For example, <c>1E9</c> renders as <c>billion</c> in <c>en-US</c> and <c>milliard</c> in <c>de-DE</c>.
+    /// </summary>
+    UseScaleWord = 16
 }

tests/Humanizer.Tests/ApiApprover/PublicApiApprovalTest.Approve_Public_Api.DotNet10_0.verified.txt

@@ -806,6 +806,7 @@ namespace Humanizer
         UseName = 2,
         UseShortScaleWord = 4,
         WithSpace = 8,
+        UseScaleWord = 16,
     }
     public class NoMatchFoundException : System.Exception
     {
@@ -1957,4 +1958,4 @@ namespace Humanizer
         public static bool TryToNumber(this string words, out long parsedNumber, System.Globalization.CultureInfo culture) { }
         public static bool TryToNumber(this string words, out long parsedNumber, System.Globalization.CultureInfo culture, out string? unrecognizedWord) { }
     }
-}
+}

tests/Humanizer.Tests/ApiApprover/PublicApiApprovalTest.Approve_Public_Api.DotNet8_0.verified.txt

@@ -805,6 +805,7 @@ namespace Humanizer
         UseName = 2,
         UseShortScaleWord = 4,
         WithSpace = 8,
+        UseScaleWord = 16,
     }
     public class NoMatchFoundException : System.Exception
     {
@@ -1956,4 +1957,4 @@ namespace Humanizer
         public static bool TryToNumber(this string words, out long parsedNumber, System.Globalization.CultureInfo culture) { }
         public static bool TryToNumber(this string words, out long parsedNumber, System.Globalization.CultureInfo culture, out string? unrecognizedWord) { }
     }
-}
+}

tests/Humanizer.Tests/ApiApprover/PublicApiApprovalTest.Approve_Public_Api.Net4_8.verified.txt

@@ -585,6 +585,7 @@ namespace Humanizer
         UseName = 2,
         UseShortScaleWord = 4,
         WithSpace = 8,
+        UseScaleWord = 16,
     }
     public class NoMatchFoundException : System.Exception
     {
@@ -1292,4 +1293,4 @@ namespace Humanizer
         public static bool TryToNumber(this string words, out long parsedNumber, System.Globalization.CultureInfo culture) { }
         public static bool TryToNumber(this string words, out long parsedNumber, System.Globalization.CultureInfo culture, out string? unrecognizedWord) { }
     }
-}
+}

tests/Humanizer.Tests/MetricNumeralTests.cs

@@ -222,4 +222,25 @@ public void ToMetric(string expected, double input, MetricNumeralFormats? format
     [InlineData(-1E-27)]
     public void ToMetricOnInvalid(double input) =>
         Assert.Throws<ArgumentOutOfRangeException>(() => input.ToMetric());
+
+    [Theory]
+    [InlineData(1E9, "1 billion")]
+    [InlineData(1E12, "1 trillion")]
+    public void ToMetric_UseScaleWord_ShortScale(double input, string expected) =>
+        Assert.Equal(expected, input.ToMetric(MetricNumeralFormats.WithSpace | MetricNumeralFormats.UseScaleWord));
+
+    [UseCulture("de-DE")]
+    [Theory]
+    [InlineData(1E9, "1 milliard")]
+    [InlineData(1E12, "1 billion")]
+    public void ToMetric_UseScaleWord_LongScale_German(double input, string expected) =>
+        Assert.Equal(expected, input.ToMetric(MetricNumeralFormats.WithSpace | MetricNumeralFormats.UseScaleWord));
+
+    [UseCulture("fr-FR")]
+    [Theory]
+    [InlineData(1E9, "1 milliard")]
+    [InlineData(1E12, "1 billion")]
+    public void ToMetric_UseScaleWord_LongScale_French(double input, string expected) =>
+        Assert.Equal(expected, input.ToMetric(MetricNumeralFormats.WithSpace | MetricNumeralFormats.UseScaleWord));
+
 }