Once a [noun], always a [noun]
[Once an X, always an X] is the short form of [once (become) an X, (will) always (be) an X]
  


"X" in this structure is a stigma or branding that cannot be removed from you. For example: "once a murderer, always a murderer" 

The label "murderer" cannot be removed from someone who committed murder. This label will be attached to him forever, because "murderer" is permanent label, you can never un-murder someone. The same goes to the label "liar". Once you are caught lying, the fact that you lied cannot be erased. 

In Chinese, it is translated as "一旦成為殺人犯，一生也是殺人犯" (once someone became a murderer, the status of murderer can never be removed from him)

you can replace "一生" with "終生" or "永遠"


  
  [Once a Y, always a Y] is the short form of [once (become) a Y, (will) always (be) a Y]
  


"Y" in this structure is a nature or attributes that cannot be taken away from you. For example: "once a soldier, always a soldier" 

Technically, when someone retired from the armed forces, he would no longed be considered a soldier, however, the attributes of a soldier will always be in him.  "Once a soldier, always a soldier" is not a literal but a figurative phrase (I am no longer a soldier, but the heart of a soldier is still in me). 

In Chinese, it is translated as "一日身為士兵，一生也是士兵" (Even being a soldier for one day, the attributes of a soldier will never be taken away from me)

you can replace "一生" with "終生" or "永遠"