  Hydrogen emission lines can provide extensive information about star-forming
galaxies in both the local and high-redshift Universe. We present a detailed
Lyman continuum (LyC), Lyman-alpha (Ly{\alpha}), and Balmer line (H{\alpha} and
H\b{eta}) radiative transfer study of a high-resolution isolated Milky-Way
simulation using the Arepo-RT radiation hydrodynamics code with the SMUGGLE
galaxy formation model. The realistic framework includes stellar feedback,
non-equilibrium thermochemistry, and dust grain evolution in the interstellar
medium (ISM). We extend our Cosmic Ly{\alpha} Transfer (COLT) code with
photoionization equilibrium Monte Carlo radiative transfer for self-consistent
end-to-end (non-)resonant line predictions. Accurate LyC reprocessing to
recombination emission requires modelling pre-absorption by dust (27.5%),
helium ionization (8.7%), and anisotropic escape fractions (7.9%), as these
reduce the available budget for hydrogen line emission (55.9%). We investigate
the role of the multiphase dusty ISM, disc geometry, gas kinematics, and star
formation activity in governing the physics of emission and escape, focusing on
the time variability, gas phase structure, and spatial, spectral, and viewing
angle dependence of the emergent photons. Isolated disc simulations are
well-suited for comprehensive observational comparisons with local H{\alpha}
surveys, but would require a proper cosmological circumgalactic medium (CGM)
environment as well as less dust absorption and rotational broadening to serve
as analogs for high-redshift Ly{\alpha} emitting galaxies. Future applications
of our framework to next-generation cosmological simulations of galaxy
formation including radiation-hydrodynamics that resolve <10 pc multiphase ISM
and <1 kpc CGM structures will provide crucial insights and predictions for
current and upcoming Ly{\alpha} observations.
