Can I translate the health certificate and the chest X-ray record to English by myself if it is originally issued in the language other than English or Japanese?

The document translated by yourself is NOT acceptable.  You need to ask your hospital again to issue one with English translation written by a doctor by explaining that you are submitting it to the oversea university.  

If they cannot, please ask the authorized translator to translate into English.  
In such case, please submit the original certificate in your language together with the one translated by the authorized translator.

If you couldn't find an answer to your question or concern,
-For enrolled students: you can inquire us through the Scomb we shared when sending you the Before Arrival Guideline.
-For prospective students: you can inquire from
here.