Optional interpolation prefix/suffix not used in plural translation cases.

Optional interpolation prefix/suffix were being used in singular, but not plural translations.

```json
{
  "app": {
    "child": "${count} child",
    "child_plural": "${count} children"
  }
}
```

```javascript
i18n.t("app.child", {
  count : 1,
  interpolationPrefix : "${",
  interpolationSuffix : "}",
});
```

Outputs "1 child"

```javascript
i18n.t("app.child", {
  count : 2,
  interpolationPrefix : "${",
  interpolationSuffix : "}",
});
```

Outputs "${count} children"