  We report on electron spin resonance (ESR) spectroscopy of boron-vacancy
(V$_\text{B}^-$) centers hosted in isotopically-engineered hexagonal boron
nitride (hBN) crystals. We first show that isotopic purification of hBN with
$^{15}$N yields a simplified and well-resolved hyperfine structure of
V$_\text{B}^-$ centers, while purification with $^{10}$B leads to narrower ESR
linewidths. These results establish isotopically-purified h$^{10}$B$^{15}$N
crystals as the optimal host material for future use of V$_\text{B}^-$ spin
defects in quantum technologies. Capitalizing on these findings, we then
demonstrate optically-induced polarization of $^{15}$N nuclei in
h$^{10}$B$^{15}$N, whose mechanism relies on electron-nuclear spin mixing in
the V$_\text{B}^-$ ground state. This work opens up new prospects for future
developments of spin-based quantum sensors and simulators on a two-dimensional
material platform.
