Hi there,
Here are some things that come to mind reading your message.
The first interview of a developer (in french) working with open data is a good start to grasp the mental model of someone coping with the shortcomings of the current data providers.
One of the main temptations when doing user research is to start drawing conclusions straight away. It is important to resist that temptation! Subsequent interviews may confirm the “mental model” you mention, or may show you something completely differnet, and it’s important to stay open to both possibilities. Try not to draw conclusions from a single interview 
this is difficult for me (and a few others in the collective) because we are not used to doing user research.
It is indeed difficult, particularly at the beginning. The good news is: things get better pretty quickly. Doing research with people is not rocket science, and quickly becomes easier the more you do it. You’ve already worked out a lot of the harder bits: you have created an interview guide, you have recruited an interviewee, you have managed to gather their consent to be interviewed, you have done the interview, you have transcribed the interview, anonymised the transcript and published it for all to see. So you have done a ton of work! With all that already in place, subsequent interviews will be much easier.
Are we overthinking this, given how simple the service is?
Certainly not. It always makes sense and always pays off to engage with the people you are designing for. You will learn a ton of stuff that will make your digital platform better in the short and in the long term.
There is a prototype with all the bits and pieces (data, documentation, release notes) but no effort whatsoever to make it usable.
Since you have this in place, you may consider using it as a prompt during your interviews, or do usability testing with it after the interviews are done. By the way, we are giving a free, practical workshop on how to do usability testing in Brussels at the end of January, just in case anybody from the collective would like attend. You can register at https://docs.google.com/forms/d/e/1FAIpQLSfCMcgZq9cG-scnVvkb7_6Z1eL1SszkPIratsdQuF0ikioDWw/viewform
Once we have a few interviews (maybe 5 from various backgrounds), should we try to get together and do an affinity mapping session?
See how you feel after the 5 interviews. If you feel you are hearing many of the same things (much commonality across all interviews), then you are ready to do some analysis. Otherwise, you will need to do some more interviews until you reach that point.
If it’s easy for some members of the collective to get together in the same physical space, an affinity diagram is a great way of working on analysis as a team. If you are not co-located, you can work on the analysis separately and in parallel, then discuss your individal analysis to draw final conclusions together. I’d be happy to provide some guidance on how to do this: just ping me when you are closer to start the analysis.
Is it worth trying to shadow a user? I tend to think the interviews are enough because the service is simple.
I would keep it simple for now. Interviews are a great way to start research because they are quite straigthforward to do. Wait to see what you learn from the interviews, then decide on next steps / research activities based on your findings.
The data sets we are working on are all in French, published by the French government, reason why the communications are also mostly in French. But these data sets are of interest to an international audience. There should be some version of the service that is available in English as well. Not all the documentation because that would be a huge undertaking. Where is the limit?
Given the potential effort involved, I would try to validate first that there is an international audience interested in the data. If there is, make the effort to translate at least the core elements of the platform. Make sure to let contributors know that you are looking for translation contributions, and put everything that needs to be translated in some kind of format people can use to submit translations (e.g. a Git repo on GitLab or similar). This is the great thing about openess: you don’t need to do everything yourselves. Interested people can help you translate the documentation!
How to get in touch with a non-french speaking developer using data sets?
Social media, forums, conferences, data-driven project mailing lists come to mind, but it really depends on who you are actually trying to reach. “Non-french speaking developers using data sets” is quite broad!
I happen to know a developer working on https://opendataservices.coop/ I could ask if he would be willing to speak to you, although I can’t guarantee anything
Any other questions you have, feel free to ask!
