framework/i18n/MessageSource.php
protected function translateMessage($category, $message, $language)
    {
        $key = $language . '/' . $category;
        if (!isset($this->_messages[$key])) {
            $this->_messages[$key] = $this->loadMessages($category, $language);
        }
        if (isset($this->_messages[$key][$message]) && $this->_messages[$key][$message] !== '') {
            return $this->_messages[$key][$message];
        } elseif ($this->hasEventHandlers(self::EVENT_MISSING_TRANSLATION)) {
            $event = new MissingTranslationEvent([
                'category' => $category,
                'message' => $message,
                'language' => $language,
            ]);
            $this->trigger(self::EVENT_MISSING_TRANSLATION, $event);
            if ($event->translatedMessage !== null) {
                return $this->_messages[$key][$message] = $event->translatedMessage;
            }
        }

        return $this->_messages[$key][$message] = false;
    }
protected function translateMessage($category, $message, $language)
    {
        $key = $language . '/' . $category;
        if (!isset($this->_messages[$key])) {
            $this->_messages[$key] = $this->loadMessages($category, $language);
        }
        if (isset($this->_messages[$key][$message]) && $this->_messages[$key][$message] !== '') {
            return $this->_messages[$key][$message];
        } elseif ($this->hasEventHandlers(self::EVENT_MISSING_TRANSLATION)) {
            $event = new MissingTranslationEvent([
                'category' => $category,
                'message' => $message,
                'language' => $language,
            ]);
            $this->trigger(self::EVENT_MISSING_TRANSLATION, $event);
            if ($event->translatedMessage !== null) {
                return $this->_messages[$key][$message] = $event->translatedMessage;
            }
        }

        return $this->_messages[$key][$message] = false;
    }
Translates the specified message.
If the message is not found, a [[EVENT_MISSING_TRANSLATION|missingTranslation]] event will be triggered.
If there is an event handler, it may provide a [[MissingTranslationEvent::$translatedMessage|fallback translation]].
If no fallback translation is provided this method will return `false`.
@param string $category the category that the message belongs to.
@param string $message the message to be translated.
@param string $language the target language.
@return string|bool the translated message or false if translation wasn't found.