DatePicker.php
private function registerTranslations()
    {
        Yii::$app->i18n->translations['datepicker'] = [
            'class' => 'yii\i18n\PhpMessageSource',
            'sourceLanguage' => 'en-US',
            'basePath' => __DIR__ . DIRECTORY_SEPARATOR . 'messages',
            'forceTranslation' => true,
        ];

        $this->clientOptions['tooltips'] = [
            'today' => Yii::t('datepicker', 'Go to today'),
            'clear' => Yii::t('datepicker', 'Clear selection'),
            'close' => Yii::t('datepicker', 'Close the picker'),
            'selectMonth' => Yii::t('datepicker', 'Select Month'),
            'prevMonth' => Yii::t('datepicker', 'Previous Month'),
            'nextMonth' => Yii::t('datepicker', 'Next Month'),
            'selectYear' => Yii::t('datepicker', 'Select Year'),
            'prevYear' => Yii::t('datepicker', 'Previous Year'),
            'nextYear' => Yii::t('datepicker', 'Next Year'),
            'selectDecade' => Yii::t('datepicker', 'Select Decade'),
            'prevDecade' => Yii::t('datepicker', 'Previous Decade'),
            'nextDecade' => Yii::t('datepicker', 'Next Decade'),
            'prevCentury' => Yii::t('datepicker', 'Previous Century'),
            'nextCentury' => Yii::t('datepicker', 'Next Century'),
            'pickHour' => Yii::t('datepicker', 'Pick Hour'),
            'incrementHour' => Yii::t('datepicker', 'Increment Hour'),
            'decrementHour' => Yii::t('datepicker', 'Decrement Hour'),
            'pickMinute' => Yii::t('datepicker', 'Pick Minute'),
            'incrementMinute' => Yii::t('datepicker', 'Increment Minute'),
            'decrementMinute' => Yii::t('datepicker', 'Decrement Minute'),
            'pickSecond' => Yii::t('datepicker', 'Pick Second'),
            'incrementSecond' => Yii::t('datepicker', 'Increment Second'),
            'decrementSecond' => Yii::t('datepicker', 'Decrement Second'),
            'togglePeriod' => Yii::t('datepicker', 'Toggle Period'),
            'selectTime' => Yii::t('datepicker', 'Select Time'),
        ];
    }
private function registerTranslations()
    {
        Yii::$app->i18n->translations['datepicker'] = [
            'class' => 'yii\i18n\PhpMessageSource',
            'sourceLanguage' => 'en-US',
            'basePath' => __DIR__ . DIRECTORY_SEPARATOR . 'messages',
            'forceTranslation' => true,
        ];

        $this->clientOptions['tooltips'] = [
            'today' => Yii::t('datepicker', 'Go to today'),
            'clear' => Yii::t('datepicker', 'Clear selection'),
            'close' => Yii::t('datepicker', 'Close the picker'),
            'selectMonth' => Yii::t('datepicker', 'Select Month'),
            'prevMonth' => Yii::t('datepicker', 'Previous Month'),
            'nextMonth' => Yii::t('datepicker', 'Next Month'),
            'selectYear' => Yii::t('datepicker', 'Select Year'),
            'prevYear' => Yii::t('datepicker', 'Previous Year'),
            'nextYear' => Yii::t('datepicker', 'Next Year'),
            'selectDecade' => Yii::t('datepicker', 'Select Decade'),
            'prevDecade' => Yii::t('datepicker', 'Previous Decade'),
            'nextDecade' => Yii::t('datepicker', 'Next Decade'),
            'prevCentury' => Yii::t('datepicker', 'Previous Century'),
            'nextCentury' => Yii::t('datepicker', 'Next Century'),
            'pickHour' => Yii::t('datepicker', 'Pick Hour'),
            'incrementHour' => Yii::t('datepicker', 'Increment Hour'),
            'decrementHour' => Yii::t('datepicker', 'Decrement Hour'),
            'pickMinute' => Yii::t('datepicker', 'Pick Minute'),
            'incrementMinute' => Yii::t('datepicker', 'Increment Minute'),
            'decrementMinute' => Yii::t('datepicker', 'Decrement Minute'),
            'pickSecond' => Yii::t('datepicker', 'Pick Second'),
            'incrementSecond' => Yii::t('datepicker', 'Increment Second'),
            'decrementSecond' => Yii::t('datepicker', 'Decrement Second'),
            'togglePeriod' => Yii::t('datepicker', 'Toggle Period'),
            'selectTime' => Yii::t('datepicker', 'Select Time'),
        ];
    }
Registers widget translations.