QTranslator *translator = new QTranslator;

translator->load("lang_en.qm");

a.installTranslator(translator);//安装翻译器

#include "widget.h"
#include "ui_widget.h"
#include  <QVBoxLayout>
#include <QTranslator>
#include <QApplication>

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    ui->setupUi(this);
    m_languageBox = new QComboBox;
    m_languageBox->addItem("Chinese","zh");
    m_languageBox->addItem("English","en");
    m_languageBox->addItem("Latin","la");
    m_languageBox->addItem("Franch","fr");

    connect(m_languageBox,SIGNAL(currentIndexChanged(int)),this,SLOT(changeLanguage(int)));

    //tr()函数用于获取需要翻译的文本

    m_nameLabel = new QLabel(tr("快乐的威猛先生"));
    m_signLabel = new QLabel(tr("人生如戏。"));

    setWindowTitle(tr("测试"));
    resize(400,100);

    QVBoxLayout* layout = new QVBoxLayout(this);
    layout->addWidget(m_languageBox);
    layout->addWidget(m_nameLabel);
    layout->addWidget(m_signLabel);
}

Widget::~Widget()
{
    delete ui;
}

void Widget::changeLanguage(int index)
{
    static QTranslator *translator;
    if(translator){
        qApp->removeTranslator(translator);
        delete translator;
        translator = nullptr;
    }

    translator = new QTranslator;

    QString languageCode = m_languageBox->itemData(index).toString();
    QString qmFile = "lang_"+languageCode;

    if(translator->load(qmFile)){
        qApp->installTranslator(translator);
    }

    //翻译文本——找不到则用逗号后面的
    m_nameLabel->setText(tr("TXT_NAME_WORD","Happy Mr. Weimeng"));
    m_signLabel->setText(tr("TXT_SIGN_WORD","Life is like a play."));
    setWindowTitle(tr("TXT_TITLE_WORD","Test"));
}