I haven't fully tested the functionality yet, but here are some comments on the UI. Sorry for the terseness - I do find the tool valuable, but the UI really needs some work.
* The tool icon is horrible and very out of the line with the default tool icons. The Tweak icon is not stellar either. Can some talented artist suggest a replacement?
* "Population" is unclear. Its tooltip does not really help. "This setting adjusts" is superfluous, just delete it, but then what remains is still confusing: "the number of items sprayed" - total number? Per click? Per mile? Per what? What does the percentage represent? Please rethink this.
* "The mean of the spray action" is impossible to understand. Mean what?
* "SD" is very techy, and while the tooltip expands it as "standard deviation", it is still confusing. Standard deviation of what value? Can you explain it in terms understandable to someone who has no math training and has no idea of what SD is.
* The button "Some options for the spray" is a bad idea, as is the dialog it opens. We don't have separate dialogs for tool options. Each tool may have some often-used controls on the controls bar, plus some rarely-used settings on its page in Inkscape Preferences. Please try to redistribute the dialog controls between these two locations. E.g. I don't think many people will ever change the distribution from Gaussian to uniform - this choice can be moved to Preferences. The controls bar can accommodate more than you have there now - if something does not fit it will be placed in an automatic menu, so just move the least used controls to the right (but preserving some logical grouping, of course). You may also try to combine some of the options into higher-level, coarser but more user-friendly preset-like options, such as a single "Scale randomization" slider instead of min/max values. (And for cursor, the Width value seems a duplicate of the controls bar's Width slider.)
Code-wise, the file spray-option.cpp contains some leftover bits from Alignment dialog (e.g. "remove overlaps" undo comment - and why do you need to make undo commit at all in this dialog?!) and some French comments that need to be translated.