Is there a one-word synonym for "less crowded" in English?

There is no solo words contextfree'ly conveying “less crowded” or “anti- crowded”; closest in this literal sense simply of “non crowded” restricted to a singular word [w/c]ould/might “uncrowded” be considered.

However, there are several words that come close in a connotative or extended (trending toward idiomatic) sense, chief among which is "quiet" (more directly opposite to "busy"), "deserted" or "empty" (meaning "completely vacant", more opposite "full") or "nearly deserted" (meaning "close to vacant", taken via context in more a relativistic sense than necessarily literal) or simply 'more than' version of "empty", namely "emptier".

The optimal single-word option to convey "much fewer people present than typical, though not necessarily zero besides self" is probably in most contexts “quiet”:

2 a : marked by little or no motion or activity : calm

followed by "emptier" as inferred by its default positive (zeroth-step) form 'empty':

1 b : not occupied or inhabited

i.e.

less-occupied/inhabited than usual

, both of which have been used with regularity in this extended semi-idiomatic sense, particularly “quiet” when ‘calmness’ i.e. non-noisiness (compared to the norm) is the deviation. Otoh, “emptier” is slightly more neutral, but closer to "non-crowded" which lacks a positively favorable connotation. If the relative lack of people is perceived as not-good, then “empt[ier/y]” might be a better choice.

That said, there are of course more than a couple multi-word and re-wording options, optimality depending on your intended usage. Chiefly among the latter I oft find myself describing some setting simply as “less crowded than usual”, “saw uncommonly few people for this time of day/week”; or there being “hardly anybody there” or “scarcely anyone present”. The reason for my pairing ‘[some]body’ with “hardly” and ‘[..]one’ with “scarcely” is subtle but real: Consider the idiom "Make youself present." —it isn't "Make your body present."—your actual “being” is closer bound to the ‘one’ness of you yourself, i.e. your personhood with all that entails, as contrasted with ‘being’ness connoting more a mere physical'ly-occupying aspect.

If you are talking about a space which is occupied only by people (at least in the context in which you are discussing it) you can say "emptier", for instance "The theatre is emptier tonight than it was on Saturday". However this has other meanings as well such as "My glass is emptier than I thought" so it's not exactly what you're looking for.

You can also say "less busy" as in "The market is less busy than it was yesterday" but that is also two words (although we can say "busier" whereas we can't say "crowdedier")

I assume from your question that there is such a word in your language but there isn't one in English. That's one of the pleasures of studying languages, not all concepts which have dedicated names in your native language have them in others and sometimes another language can have a name for a concept which doesn't exist in your own.