Can I make a DCP where the projectionist can switch subtitles on or off, or choose the language?

No. There isn't, in general, an easily-accessible control for the projectionist to enable/disable subtitles, or to choose which language they should be in. One of the core ideas with DCP is that a particular CPL (Composition Playlist) should always look and sound the same. The only simple control a projectionist usually has to affect the presentation is a volume knob.

To allow your film to be shown with or without subtitles you need to make two DCPs. First, make a “OV” (original version) DCP without subtitles. Then you can use the VF feature to make a second “VF” (version file) DCP which adds the subtitles. The VF DCP will be very small, as it only contains the subtitles, and refers to your first DCP to get the picture and sound.

If you want a single folder to send to the cinema you can merge the two DCPs you made into one using the DCP-o-matic Combiner tool.

You can do similar things to allow a choice of subtitle languages. Start with an OV DCP with no subtitles, then make a new DCP for each of your subtitle languages.