You will have to hack the code (vm/soy files) in the JIRA Installation to change the "More" and "Assign" buttons (if you are talking about the system menu buttons).
Note: You can hide the native "Assign" button if you wish https://confluence.atlassian.com/display/JIRAKB/How+to+remove+the+'Assign'+button+from+issues
Can you explain more about what you are trying to achieve here? It is correct that these buttons are not translatable through the InProduct Translation plugin but there may be another way to deal with the Assign option by moving it to a workflow action where you can give it any name you like.
Unfortunately I do not know of a similar trick for the More button.
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