Does ext4 / cifs system need kernel NLS support ? I'm not sure if it's being handled by user-space program (decoding / encoding) ?
NLS allows normalization of character sets used for filenames over the whole system, so you can have different charset used on two different systems and still have correct mappings.
So yes, it's necessary, especially for CIFS, which afaik uses Unicode by default on newer servers, but your local system might have different settings (usually UTF-8 these days, fortunately).
Unfortunately, applications don't handle that (and why should they?).