OK, we'll need to debug2 error_log file to see what is coming from the browser and what is getting passed into the CGI program. Please change the log level, view the jobs page, and then send us the error_log file.
I have tried CUPS r5357, but no changes. Dates are in English.
I have used three browsers: Firefox 126.96.36.199, Mozilla 1.7.10 and Konqueror 3.5.1.
I have tried differents browser locale settings putting as first option Spanish/Spain (es-es) or Spanish (es) with the same result.
Of course, I installed r5357 yesterday. I have booted my computer two times today. I restarted cupsd many times.
I have run ldconfig and after that I have stopped/started cupsd. The first thing I see is that home page is in English (!). Other pages are mixed (buttons in English and the rest in Spanish). But the date are OK at last.