Uncaught syntaxerror: invalid or unexpected token

That's a javascript error and javascript runs off what is sent to the browser (not what is in your cshtml file) so look at the page source.

var path = '@Url.Action("index","Survey")?id='
           + @Model.MyGuid+'&languageName=' +selectedValue;
       alert(path.val());

The above will render as something like this

var path = '/Survey/index?id='
           + {6AAE15E3-E702-4251-A6F1-2A27EBBAA4A0}+'&languageName=' +selectedValue;
       alert(path.val());

Is that valid javascript? No. When it gets to "+ {" it thinks the { is a code block. Now look at your working example

var t = "@Url.Action("index","Survey")?languageName="
                 + selectedValue +"&id=@Model.MyGuid";

View the source and it will be like

var t = "/Survey/index?languageName="
                 + selectedValue +"&id={6AAE15E3-E702-4251-A6F1-2A27EBBAA4A0}";

Is that valid? Yes. The "{ }" is inside a string block (it has quotes around it) so it knows it is a literal string and not a code block.

That might not be the specific issue as we don't know what MyGuid renders as, but it's going to be related to the same root issue.

var path = '@Url.Action("index", "Survey")?id=@Model.MyGuid&languageName=' + encodeURIComponent(selectedValue);

var path = '@Url.Action("index", "Survey", new { id = Model.MyGuid })&languageName=' + encodeURIComponent(selectedValue);