It would depend on context. 'Less effort' would be the most common e.g. 'learning a language similar to your own takes less effort than learning one that is vastly different' 'Fewer efforts' might be used in sentences like: 'Country A has implemented a number of measures to address this problem while Country B has made substantially fewer efforts to fix the same problem' (so effort here refers to specific measures e.g. 'relief efforts'). So where effort is uncountable you would use 'less effort' and where efforts are countable you would use 'fewer efforts'. Now 'less efforts' is not grammatically correct, less is only meant to be used for things which are not countable e.g. less water, less air etc. This is why less corresponds to the uncountable 'effort'. Although less is occasionally used by native speakers when talking about countable objects it is incorrect e.g. express checkout counters at shops for people with '10 items or less' (it should really be 10 items or fewer).

"less effort" since effort is usually uncountable

作“吃力，费力”讲时为不可数；作“（一系列）行动，活动”讲时为可数；作“努力”讲时既可数又不可数，随便用。