diff -r 9e6f7cda68d1 poi-3.15/src/main/java/org/apache/poi/ss/format/CellNumberFormatter.java
--- a/poi-3.15/src/main/java/org/apache/poi/ss/format/CellNumberFormatter.java	Thu Mar 09 15:46:04 2017 +0100
+++ b/poi-3.15/src/main/java/org/apache/poi/ss/format/CellNumberFormatter.java	Thu Mar 09 16:06:27 2017 +0100
@@ -26,6 +26,7 @@
 import java.util.Iterator;
 import java.util.List;
 import java.util.ListIterator;
+import java.util.Locale;
 import java.util.Set;
 import java.util.TreeSet;
 
@@ -259,8 +260,7 @@
             }
             fmtBuf.append('E');
             placeZeros(fmtBuf, exponentSpecials.subList(2, exponentSpecials.size()));
-            DecimalFormatSymbols dfs = DecimalFormatSymbols.getInstance(LocaleUtil.getUserLocale());
-            decimalFmt = new DecimalFormat(fmtBuf.toString(), dfs);
+            decimalFmt = new DecimalFormat(fmtBuf.toString(), DecimalFormatSymbols.getInstance(Locale.US));
             printfFmt = null;
         }
 
@@ -444,9 +444,9 @@
             writeFraction(value, null, fractional, output, mods);
         } else {
             StringBuffer result = new StringBuffer();
-            Formatter f = new Formatter(result, LocaleUtil.getUserLocale());
+            Formatter f = new Formatter(result);
             try {
-                f.format(LocaleUtil.getUserLocale(), printfFmt, value);
+                f.format(Locale.US, printfFmt, value);
             } finally {
                 f.close();
             }
@@ -538,6 +538,13 @@
             toAppendTo.append('-');
         }
         toAppendTo.append(output);
+        final DecimalFormatSymbols dfs = DecimalFormatSymbols.getInstance(LocaleUtil.getUserLocale());
+        final String finalized = toAppendTo.toString()
+        		.replaceAll("[.]", "!")
+        		.replaceAll(",", String.valueOf(dfs.getGroupingSeparator()))
+        		.replaceAll("!", String.valueOf(dfs.getDecimalSeparator()));
+        toAppendTo.setLength(0);
+        toAppendTo.append(finalized);
     }
 
     private void writeScientific(double value, StringBuffer output, Set<CellNumberStringMod> mods) {
@@ -698,9 +705,9 @@
     private void writeSingleInteger(String fmt, int num, StringBuffer output, List<Special> numSpecials, Set<CellNumberStringMod> mods) {
 
         StringBuffer sb = new StringBuffer();
-        Formatter formatter = new Formatter(sb, LocaleUtil.getUserLocale());
+        Formatter formatter = new Formatter(sb);
         try {
-            formatter.format(LocaleUtil.getUserLocale(), fmt, num);
+            formatter.format(Locale.US, fmt, num);
         } finally {
             formatter.close();
         }
@@ -811,6 +818,13 @@
      */
     public void simpleValue(StringBuffer toAppendTo, Object value) {
         SIMPLE_NUMBER.formatValue(toAppendTo, value);
+        final DecimalFormatSymbols dfs = DecimalFormatSymbols.getInstance(LocaleUtil.getUserLocale());
+        final String finalized = toAppendTo.toString()
+        		.replaceAll("[.]", "!")
+        		.replaceAll(",", String.valueOf(dfs.getGroupingSeparator()))
+        		.replaceAll("!", String.valueOf(dfs.getDecimalSeparator()));
+        toAppendTo.setLength(0);
+        toAppendTo.append(finalized);
     }
 
     private static Special lastSpecial(List<Special> s)  {