[#8557] Standardize locale

src/main/java/teammates/common/util/StringHelper.java

@@ -1,6 +1,7 @@
 package teammates.common.util;
 
 import java.text.DecimalFormat;
+import java.text.DecimalFormatSymbols;
 import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
@@ -179,7 +180,9 @@ public static <T> String toString(List<T> list, String delimiter) {
      * Converts a double value between 0 and 1 to 3dp-string.
      */
     public static String toDecimalFormatString(double doubleVal) {
-        DecimalFormat df = new DecimalFormat("0.###");
+        DecimalFormatSymbols syms = new DecimalFormatSymbols();
+        syms.setDecimalSeparator('.');
+        DecimalFormat df = new DecimalFormat("0.###", syms);
         return df.format(doubleVal);
     }
 

src/main/java/teammates/common/util/TimeHelper.java

@@ -7,6 +7,7 @@
 import java.time.ZonedDateTime;
 import java.time.format.DateTimeFormatter;
 import java.time.format.DateTimeParseException;
+import java.util.Locale;
 
 /**
  * A helper class to hold time-related functions (e.g., converting dates to strings etc.).
@@ -105,7 +106,9 @@ public static String formatInstant(Instant instant, String timeZone, String patt
         if (zonedDateTime.getHour() == 12 && zonedDateTime.getMinute() == 0) {
             processedPattern = pattern.replace("a", "'NOON'");
         }
-        DateTimeFormatter formatter = DateTimeFormatter.ofPattern(processedPattern);
+        DateTimeFormatter formatter = DateTimeFormatter
+                .ofPattern(processedPattern)
+                .withLocale(Locale.US);
         return zonedDateTime.format(formatter);
     }