Translation:The girls worked in the garden on Saturday.
Děvčata are physically females. But the word děvče is neuter gender (just like Mädchen in german) so that's why děvčata pracovala is correct.
So 'na' here seems to refer to the location, so the girls do not necessarily do gardening, the garden is just where they worked. But can 'na' also refer to the project, e.g. 'The girls worked on the garden on Saturday', so the girls did gardening.
Unfortunately in this phrase, there's no distinction in English between the location and the work being done; the phrase "in the garden" is used for both. Normally if they are in the garden but are doing something other than gardening, the way to clarify it is to state what the other work is; for example, "they were in the garden working on the school project."
I have the same question about pracovala and pracovaly or pracovali. I always forget which is the plural of girls
Pracovali - plural for masculine animate, a group of men or a mixed group (men+women)
Pracovaly - plural for feminine, masculine inanimate
Pracovala - singular for feminine, plural for neuter gender.
your translation should actually be for: On Saturday the girls worked in the garden
"On Saturday, the girls worked in the garden" is also accepted -- along with 672 other options. But only one can be shown at the top of the discussion page.
My question is about the lack of definite article. In English, 'Girls worked ...' is not the same as 'The girls worked ...'. The latter implies a specific set of girls. Does this cause confusion in Czech?