Is the problem that this if uses the "host" plarform instead of the "target" platform.

Yes, when compiling with SLINT_FEATURE_FREESTANDING, we still pick the host platform.

The equivalent in rust

slint/internal/common/lib.rs

Line 16 in 9650aa0

pub fn get_native_style(has_qt: bool, target: &str) -> &'static str {

uses the target platform.
I guess then that would be equivalent because fluent is the callback.

More specifically, it uses the target_os, which doesn't exist with SLINT_FEATURE_FREESTANDING (or no_std).

But we would still have the problem when cross building an UI for an embedded linux device, wouldn't we?

Cross-compiling for an embedded Linux device will hopefully not end up using SLINT_FEATURE_FREESTANDING/no_std but a cmake toolchain file and the existing selection will work.