Tweak a localization to remove any assumption on "+" localization

See discussion at
#19472 (comment)

WordPress/Classes/ViewRelated/Stats/Insights/ViewsVisitors/ViewsVisitorsLineChartCell.swift

@@ -57,30 +57,45 @@ struct StatsSegmentedControlData {
         // We want to show something like "+1.2K (5%)" if we have a percentage difference and "1.2K" if we don't.
         // Because localized strings need to be strings literal, we cannot embed any conditional logic in the `localizedString...` call.
         // We therefore need to generate different string literals based on the state.
-        let differenceSign = difference < 0 ? "" : "+"
-        if differencePercent != 0 {
-            let stringFormat = NSLocalizedString(
-                "insights.visitorsLineChartCell.differenceLabelWithPercentage",
-                value: "%@%@ (%@%%)",
+        switch (difference > 0, differencePercent != 0) {
+        case (true, true): // E.g.: +1.2k (5%)
+            stringFormat = NSLocalizedString(
+                "insights.visitorsLineChartCell.differenceLabelPosiviteWithPercentage",
+                value: "+%@ (%@%%)",
                 comment: "Difference label for Insights Overview stat, indicating change from previous period, including percentage value. Example: +99.9K (5%)"
             )
             return String.localizedStringWithFormat(
                 stringFormat,
-                differenceSign,
                 difference.abbreviatedString(),
                 differencePercent.abbreviatedString()
             )
-        } else {
-            let stringFormat = NSLocalizedString(
-                "insights.visitorsLineChartCell.differenceLabelWithoutPercentage",
-                value: "%@%@",
+        case (true, false): // E.g.: +1.2k
+            // We cannot assume every locale would translate an English string like "+1.2k" in the same way.
+            // So, even though we have only a "+" prefix, we ought to make this string localized.
+            stringFormat = NSLocalizedString(
+                "insights.visitorsLineChartCell.differenceLabelPosiviteWithoutPercentage",
+                value: "+%@",
                 comment: "Difference label for Insights Overview stat, indicating change from previous period. Example: +99.9K"
             )
             return String.localizedStringWithFormat(
                 stringFormat,
-                differenceSign,
                 difference.abbreviatedString()
             )
+        case (false, true): // E.g. 1.2k (5%)
+            stringFormat = NSLocalizedString(
+                "insights.visitorsLineChartCell.differenceLabelNotPosiviteWithPercentage",
+                value: "%@ (%@%%)",
+                comment: "Difference label for Insights Overview stat, indicating change from previous period, including percentage value, when the change is 0 or less. Example: 99.9K (5%)"
+            )
+            return String.localizedStringWithFormat(
+                stringFormat,
+                difference.abbreviatedString(),
+                differencePercent.abbreviatedString()
+            )
+            break
+        case (false, false): // E.g.: 1.2k
+            // There's no + sign nor percentage value here, we don't need to add any localization treatment.
+            return difference.abbreviatedString()
         }
     }