Hello Ava,
Searching a bit I can't see any Cordova plugin that allows to the user to choose a location in order to save a file. We can see some plugins that allows to the user to choose a file, but, apparently, there is not a plugin to select a place to "save a file".
A possible approach can be to save the file in a public location, that is, instead of a private location, only available for the app. In other words, we can save the file into the root of the device's "sdcard", so we can then inform to the user in which place the file can be found: since the file is saved in a public location, the user can locate it without problems.
Certainly we can always use the appropriate Javascript, for example, the below code save a "sample.txt" file with the contents "Hello world!", in the device's "sdcard" root directory. Note how an alert dialog is shown to the user when the work end: "The file has been successfully saved at: ...":
The above Javascript code is perfectly ready to use. Additionally, the FileRead, FileWrite and FileAppend actions of App Builder has been enhanced, and now, we can specify any of the available directories in FileRead, and, a new "Directory" argument has been added to the FileWrite and FileAppend actions, to allows to specify any of the available directories.
So, if you upgrade your App Builder copy, you can do the same than the above with the enhanced FileWrite action, since this (and FileAppend) allows now to specify the "externalRootDirectory", instead to use by default the app's data directory, which is private to the user. For information about what directory to use in what platform, you must read the documentation of the Apache Cordova Files plugin.
So, in few words, replying to your question, I think the approach must be to save the CSV file in a public directory, like the "externalRootDirectory" in Android, so you can inform to the user that the CSV file has been placed in that directory, instead to ask the user where he wants the file to be saved.