var themeStore = Ext.create('Ext.data.Store', {
fields: ['value', 'name'],
data: [
{ value: 0, name: "default" },
{ value: 1, name: "access" },
{ value: 2, name: "gray" },
{ value: 3, name: "scoped" }]
});

var languageStore = Ext.create('Ext.data.Store', {
fields: ['value', 'name'],
data: [
{ value: "en", name: "English" },
{ value: "zh", name: "Chinese"}]
});

items: [{
xtype: 'combobox',
name: 'Language',
queryMode: 'local',
maxLength: 16,
fieldLabel: 'Language',
displayField: 'name',
valueField: 'value',
//value: 'en',
editable: false,
store: languageStore
}, {
xtype: 'combobox',
name: 'Theme',
queryMode: 'local',
maxLength: 16,
fieldLabel: 'Theme',
displayField: 'name',
valueField: 'value',
value: 0,
editable: false,
store: themeStore
}]

form.findField('Language').select('en');

form.findField('Theme').select(0);

As below

noname.png