  We report on the geometrically frustrated two-dimensional triangular-lattice
magnets $A_2$La$_2$NiW$_2$O$_{12}$ ($A$ = Sr, Ba) studied mostly by means of
neutron powder diffraction (NPD) and muon-spin rotation and relaxation
($\mu$SR) techniques. The chemical pressure induced by the Ba-for-Sr
substitution suppresses the ferromagnetic (FM) transition from 6.3 K in the
Ba-compound to 4.8 K in the Sr-compound. We find that the $R\bar{3}$ space
group reproduces the NPD patterns better than the previously reported
$R\bar{3}m$ space group. Both compounds adopt the same magnetic structure with
a propagation vector $\boldsymbol{k} = (0, 0, 0)$, in which the Ni$^{2+}$
magnetic moments are aligned ferromagnetically along the $c$-axis. The
zero-field {\textmu}SR results reveal two distinct internal fields (0.31 and
0.10 T), caused by the long-range ferromagnetic order. The small transverse
muon-spin relaxation rates reflect the homogeneous internal field distribution
in the ordered phase and, thus, further support the simple FM arrangement of
the Ni$^{2+}$ moments. The small longitudinal muon-spin relaxation rates, in
both the ferromagnetic- and paramagnetic states of A$_2$La$_2$NiW$_2$O$_{12}$,
indicate that spin fluctuations are rather weak. Our results demonstrate that
chemical pressure indeed changes the superexchange interactions in
$A_2$La$_2$NiW$_2$O$_{12}$ compounds, with the FM interactions being dominant.
