I agree that the Death Guard aren't as easy to pick names for as many of the other Legions! Still, looking through the names in Collected Visions certainly helps give a feel of the sort of names that seem prevalent (presumably among the members of the Legion recruited on Barbarus, rather than the Terran marines).
e.g. Kargul; Sorrak; Holgoarg; Grulgor; Gurtur-Fol; Mortag; Moturg; Rorrorg; Ruggoth-Kal; Haldon-Tal; Kalton-Gul; Guorzlat etc...
While it doesn't seem drawn from any recognisable historical/national source as far as I can see, there's a definite pattern! Harsh sounding names, plenty of plosive sounds/letters, glottoral stops, and the like. Using names with plenty of Gs, Ks, Ts, adding -Tal, -Gul, or similar to the end of names, and ensuring they sound fairly harsh. They seem (to me at least) to be somewhat Germanic-sounding (while not actually being German names!)
Anyway, requires a bit more work than (for example) raiding the bible for the names of angels, but hopefully should provide a starting point at least.