Thanks for the PR. I have been thinking of doing something like this for a while, mainly because I haven't been able to find a good way to have the dialog name show in the system language on Linux, whether with GTK or portal.

The main issue with this feature is API and ABI breakage. This PR will break ABI in all interfaces and break API in the plain C interface (nfd.h), and NFDe doesn't really have a good transition path to perform ABI and API breaking changes. I would be interested to know what some of the more popular projects that depend on NFDe think about this.

Related: mlabbe/nativefiledialog#104

This won't be merged as is, due to ABI and API breakage. I intend to add support for custom titles for all dialogs in a new API, described in #92, and it would be totally new code different from what you have done. Are you still interested in contributing?