So you need to follow the instructions on how to change/update translations.
But you need to know, if you change the translation of an existing translation (e.g. Chinese) any update can overwrite your changes. If it is an error in translation, you might suggest a change to the translation.
If it is a specific change you want to make to adapt the text better to your specific needs, you might create a new language (e.g. chinese-ming) and use that instead for your survey.
I answer at the LimeSurvey forum in my spare time, I'm not a LimeSurvey GmbH employee.
No support via private message.
By the way: Seeing that you are using version 2.50 160616 I think it is safe to assume that you are not updating your installation on a regular basis anyway. So you are probably "safe" to change the language file directly.
But you might be very unsafe because your installation hasn't received any security updates for almost 2 years now.
I answer at the LimeSurvey forum in my spare time, I'm not a LimeSurvey GmbH employee.
No support via private message.