I need help with website development

Very briefly, my project is to create an online utility that would allow users to enter the modern geographical hierarchy of any village in Germany along with a year (between roughly 1600 and 1800) and have the utility return the geographical hierarchy that existed in the selected year. (see https://upload.wikimedia.org/wikipedia/commons/7/7d/Holy_Roman_Empire_1648.svg for a taste of the challenge I have set for myself.)

The front end would consist of a series of drop-down menus, with each menu being contingent upon the selection made in the previous one. I have found open-source code that generates a drop-down menu with US states and then a second menu with a list of counties in the selected state. I have modified that to replace US states with German states and counties with “Regierungsbezirks”, but it doesn’t work because I am working off my C drive and it generates CORS errors. So my first issue is figuring out what environment I should be working in to avoid that. (My front end will have a total of 5 drop-down menus, which I assume I will be able to generate myself using the existing code once I am working in the right environment. I also expect to be able to add the box for the year by myself.

I have written the back end in Fortran because that is the language I am comfortable with. It is a simple program, about 250 lines long. I have also found open-source code that purports to allow the Fortran output to connect to the front end. I have not tried to implement it because I assume I will encounter the same CORS errors I am already getting. I am not committed to keeping the backend in Fortran because I understand that hosting sites frequently do not allow .exe files. Furthermore, the Fortran is not handling German characters correctly. So I need some advice on how I should modify the backend or, alternatively, I need somebody to rewrite it in a different language and hook it up to the front end.

Most urgently, I need to establish the format in which I should keep my data files. When complete, there will be literally tens of thousands of data files that the utility will need to access. At the moment, I am keeping everything in Excel and writing out .prn files for the Fortran to read (Fortran doesn’t play well with .csv files). I already have a lot of those files and I don’t want to get too far down the road and then have to reformat them all.

In the long run, the utility will be one prong of a two-pronged website. The other prong will be entirely in HTML and will allow users to see historical geographies with more context than a single village (e.g., lists of villages in each county-equivalent, lists of county-equivalents in each country, etc.). I have undertaken a similar project in the past (much of which I will incorporate into this one), and I am convinced that I can handle the programming myself, but my earlier project was ugly and I could use some help coming up with some design standards that would make it more aesthetically pleasing.