Can you please post the patch you are trying to apply and what has been currently applied.
Actually , I came across this situation when my friend had asked this question few weeks back.
Thought to get some idea on this issue.
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I personally would recommend to have the pre-requisite patches to have the respective languages also applied.
So, for applying the current patch X do I have to install the french patches for the already installed pre-req's also or shall I skip them ?
Yes, you have to install pre-req. patches even for NLS ones.
Requesting Translation Synchronization Patches (Doc ID 252422.1)