Is my character who worships Gruumsh more likely to speak Infernal, Celestial, or Abyssal?

Question

I'm making a half-orc cleric that worships Gruumsh, and the half-orc knows how to speak the language that Gruumsh speaks(besides Common/Orc). Would he speak Celestial, or Infernal/Abyssal?

Answer 1

Abyssal or Infernal would be the more appropriate additional language

As a god, Gruumsh could speak whatever language he wants. If your character knew how to speak any language Gruumsh can speak, that would mean they can speak all of them. Unless your DM wants to allow your character to speak all languages, if you need to pick one secondary language, Infernal or Abyssal would be more reasonable picks than Celestial.

Gruumsh is the patron deity of orcs, and his alignment is given as Chaotic Evil (PHB, p. 296). This would make Abyssal the most probable language to use¹, the language of the chaotic evil demons, all of which are at home in evil-aligned lower planes. The Tanarukk, which is a demon orc (Volo's Guide to Monsters, p. 186) likewise speaks Abyssal, Common, and Orc.

Gruumsh lives in Acheron. Volo's Guide to Monsters (p. 82):

[...] the orcs know that if they fight well and bring glory to their tribe, Gruumsh will call them home to the plane of Acheron. It is there in the afterlife where the chosen ones will join Gruumsh and his armies [...]

Acheron is an eternal battleground between orcs and hobgoblins and Infernal would be common there. (Thanks to @MattVincent for finding this; see his answer for more detail).

In contrast, Celestial is the language of the celestials (such as angels), who are at home in good-aligned upper planes – much worse a fit. (See PHB page 123 for a list of languages.)

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¹ Older editions – starting from first edition – gave Gruumsh as lawful evil, with the Nine Hells as his home plane. That would make Infernal better, but this is not the case any more in 5e.

Answer 2

It's a DM call – but Gruumsh lives in Acheron, where historically a lot of creatures spoke Infernal

Volo's Guide to Monsters (p. 82) says this about orcs:

[...] the orcs know that if they fight well and bring glory to their tribe, Gruumsh will call them home to the plane of Acheron. It is there in the afterlife where the chosen ones will join Gruumsh and his armies [...]

Historically, many of the inhabitants of "the Infernal Battlefield of Acheron" spoke Infernal (and I've found no 5e material conflicting with this). The Manual of the Planes (3.5e, p.124) stated that in addition to the spirit legions:

Achaierai, devils, imps, fomorians rakshasas, dragons and yugoloths also inhabit Acheron

All of these except dragons & fomorians were explicitly listed as speaking infernal. Maybe it's because these beings can easily reach Acheron via the river styx, while Demons cannot (as they'd be stopped in Avernus by Devils).

Also fwiw (in editions 1 through 3.5): Achaierai are the only known true natives to Acheron, and they have always only spoke infernal .

Answer 3

Gruumsh understands and speaks all languages.

The 3e supplement Deities and Demigods has a section on Gruumsh, which lists his special qualities (p. 71):

Special Qualities: Orc traits, divine immunities, DR 67/+5, fire immunity, spontaneous casting of divine spells, understand, speak, and read all languages

Nothing in 5e has this level of specificity, so for this detail, you have to look to the older material.

So it doesn’t matter what your character speaks; Gruumsh can understand it. So instead of focusing on Gruumsh for this choice, focus instead on your background to find your third language.