Handle specific german letters like ä, ö, ü

Handle specific german letters like ä, ö, ü

I have a little problem with my sencha touch app's SignIn-Function. It
seems like it cannot handle some german letters called "Umlaute" like 'ä',
'ü','ö' and 'ß'. By placing some information recieved by the server into
any view, it doesn't present the letters ä ö ü correctly.
Thank you very much for help in advance :)
onSignInCommand: function(view, username, password) {
var me = this;
loginView = me.getLoginView();
var data = Ext.JSON.encode({
"method": "getUserInfo",
"params": [username,password]
});
Ext.util.JSONP.request({
url: 'http://localhost',
headers: {
'content-type': 'application/x-www-form-urlencoded ;
charset=utf-8'
},
method: 'post',
params: {
"json": data
},
timeout: '4000',
callbackName: 'myCallback',
success: function (response) {
var loginResponse = response;
if (loginResponse.result.msg == "OK")
{
var userstore = Ext.data.StoreManager.lookup('myStore');
var userdata=response.result.user;
var usernamestore =
Ext.data.StoreManager.lookup('usernamestore');
var tokenstore = Ext.data.StoreManager.lookup('tokenstore');
var myusername = loginResponse.result.user.login;
var mytoken = loginResponse.result.user.token;
var records= userstore.getRange();
userstore.remove(records);
userstore.add(userdata);
var jsonArray = userstore.data.items;
usernamestore.add(myusername);
tokenstore.add(mytoken);
me.signInSuccess(myusername,mytoken);
var usertest = jsonArray[0].data.login;
var tokentest = jsonArray[0].data.token;
}
else{
loginView.showSignInFailedMessage('token null.');
}
},
failure: function (response) {
loginView.showSignInFailedMessage('token null.');
}
}
);
},