The word less can be an adjective and an adverb. It is an adjective when it goes before a noun. As an adjective, less is used before an uncountable noun.
Example: less money
- I earn less money than my brother. (NOT I earn fewer money…)
When less goes before an adjective, it becomes an adverb modifying that adjective.
In the expression ‘less popular languages’, the adjective popular modifies the noun languages and the adverb less modifies the adjective popular.
When less is an adverb, the countable – uncountable rules do not apply because adjectives are neither countable nor uncountable.
- She is less popular than me.
A lot of English words can be adjectives and adverbs. Just look at the placement. If less goes before a noun, it is an adjective. If it goes before an adjective, it is an adverb.