Normally, when the argument of a Twig Function is a Callable, Twig creates such call (see Twig's `compileCallable()`).
Eg:
```
{{__("foo")}}
=> echo twig_escape_filter($this->env, call_user_func_array($this->env->getFunction('__')->getCallable(), array("foo")), "html", null, true)
```

This has the side effect that effect resulting template loose the xgettext friendly syntax.

By using feeding string to Twig Function, the result is clearer (to xgettext):
```
{{__("foo")}}
=> echo twig_escape_filter($this->env, __("foo"), "html", null, true);
````

Since wrapping these functions is of no use (same arguments), and since compiled templates containing these functions
 may very well have to be looked into for gettext*() function calls, it's better to avoid it.