I have put those volumes in the excel attached. Looks like most of the CIFS Volumes are set to 'C POSIX', instead of 'C.UTF-8'. Strangely, root volumes are C.UTF8, so are they explicitly set to 'C' during creation.
According to cDOT: You can specify the language for a volume when creating a volume and it can be different from the language of an SVM. If you do not specify the language for a volume then it inherits the language setting of its SVM. After the volume is created, you cannot modify the language of a volume. Therefore, you must be aware of the available language options.
I guess, you could modify the SVM Language, however that will only allow 'NEW' volumes to inherit the language settings. There isn't much information around why it cannot be done, but I believe you need to just weigh-off options, what is more priority. As I had mentioned, there is free monitoring tool from NetApp, which you can for health/performance related stuff, however if you would like to change the volume then as I see, you will have to create a new Volume with correct language setting and they copy the data over. Is it worth it?
List of language options:
https://docs.netapp.com/ontap-9/index.jsp?topic=%2Fcom.netapp.doc.dot-ivg%2FGUID-8420C0E3-4208-4CD0-AAC6-2197C10FE9F7.html