Description
Unfortunately, the translations used in the Windows installer are not in a standard form that Weblate can handle. They are currently maintained in a Inno Setup Script file https://github.com/r-devel/r-svn/blob/41e65b267f382e811458045857739899f8cb6be5/src/gnuwin32/installer/CustomMsg.iss, which defines custom messages for each language. These are imported after the standard translation files in https://github.com/r-devel/r-svn/blob/41e65b267f382e811458045857739899f8cb6be5/src/gnuwin32/installer/header1.iss#L45.
Looking at the InnoSetup docs, it looks like we might be able to define a custom message file for each language (https://jrsoftware.org/ishelp/index.php?topic=languagessection), something like
[Languages]
Name: "en"; MessagesFile: "compiler:Default.isl,compiler:en.islu"
Name: "nl"; MessagesFile: "compiler:Languages\Dutch.isl,compiler:Languages\nl.islu"
The .islu format is a Unicode version of the .isl format, and this is supported by Weblate: https://docs.weblate.org/en/latest/formats.html#islu.
In order to build the Windows installer, it looks like you need the full setup to build R on Windows, so this is a non-trivial task to test out. However, if we could get it working, then it should not be too hard to add these strings to Weblate, based on our experience with r-devel/r-project-sprint-2023#31.
I would be happy to help someone get set up building R on Windows on their own laptop and to work on this together.
References
Howto: Building R-devel and packages on Windows - official docs, including building Windows installer.
Building R > Windows - simpler step-by-step just for building R.