//显示'全选'和'全不选'
function selectAllDisplay(flag=true){ 
    if (flag){
        selectAllLabel.hide();
        deselectAllLabel.show();
    }
    else{
        selectAllLabel.show();
        deselectAllLabel.hide();
    }
    selectAll[0].checked=flag; //这里会冗余执行
}
//全选或全不选
function selectAllOrNot(flag=true) {
    langs.prop('checked',flag);
    selectAllDisplay(flag);
}
//判断是否全选
function isSelectAll() {
    var flag=true;
    langs.each(function (){flag=flag&&this.checked;})
    return flag;
}
//全选click触发函数而不是change
selectAll.click(function () {
    this.checked?selectAllOrNot(true):selectAllOrNot(false);
})

langs.change(function (){
    isSelectAll()?selectAllDisplay(true):selectAllDisplay(false);
})

invertSelect.click(function (){
    langs.each(function (){this.checked=!this.checked;});
    langs.change();
})