On the Monster Manual, the 3.5 revision, and bonus languages
The Third Edition original Monster Manual (2000) includes verbatim the section the question quotes on Intelligence (10), but that Monster Manual itself makes no mention of bonus languages. In Dungeons & Dragons, Third Edition—prior to the 3.5 revision—bonus languages are a game element exclusive to the Player's Handbook (2000). Further, like in Advanced Dungeons & Dragons, 2nd Edition and earlier editions, the Monster Manual for Third Edition isn't designed as a player-facing reference but for the DM. Little truck is given to PC–NPC transparency except insofar NPCs that are of PC-suitable races follow the rules like a PC would; for example, NPC monsters that are advanced by Hit Dice are advanced totally differently from PCs, gaining feats and skill points at rates completely different from PCs' rates.
(To give the reader an idea of how this works, the question's typical slaughterking kython possesses 18 HD and the DM can advance it to up to 36 HD. In Third Edition, according to the Monster Manual's proprietary advancement formula for creatures possessing the type aberration (13) that advancement of 18 HD sees the slaughterking kython gain 36 skill points and 4 extra feats. However, after the 3.5 revision there's truer PC–NPC transparency, and the slaughterking kython from advancing that same 18 HD gains 90 skill points and 5 extra feats just as an 18 HD Int 20 PC would were that PC to gain another 18 HD.)
To this reader it appears that the game was originally designed so that an NPC that's of a race that's ill-suited for use as a PC receives extra languages according the NPC's high Intelligence score, and the DM can pick any languages for the NPC's extra languages—even secret languages like druidic and githyanki. (This reader imagines the developers discussing this prospect and concluding If a DM wants some brainy ogre to speak Druidic, it's his campaign.)
It was only with the 3.5 revision of the Monster Manual and its addition of player-facing material that saw those additions possibly create an unintentional cascade throughout the system of highly intelligent yet strangely (given how most PCs who aren't warforged function) monolingual or limited-languages-known creatures. (A situation exemplified by this fine answer.)
That is, prior to the 3.5 revision, "[c]elestials [like the solar] speak Celestial, Infernal, and Draconic" (31). Likewise, "all devils [including the pit fiend] speak Infernal, Celestial, and Draconic" (50). However, while this is largely unchanged by the 3.5 revision (see Monster Manual (2003) 10 and 51, respectively), the lack of a Bonus Languages entry for the solar could mean it misses out on six extra languages and the same absence could mean the pit fiend misses out on eight extra languages.
(Note that solars possess a continuous effect that's like the tongues spell and pit fiends possess the ability to telepathically communicate with any creature that possesses a language that's within 100 ft. of them. However, neither ability allows the creature to read a language. A solar could've prepared a comprehend languages spell if it expected to need to read Common or whatever, but the typical, straight-from-the-Monster Manual (2003) pit fiend that has Intelligence 26 (!), by default, may not even be able to read a to-go menu if it's written in Common!)
What to do?
This DM is always on the lookout for ways to make monsters more than just loot piñatas, so with all of this in mind this DM recommends a DM give those creatures that are listed as speaking at least one language additional languages according to their high Intelligence scores just like both the Third Edition of the Monster Manual and the 3.5 revision of the Monster Manual say to do except in cases wherein the 3.5 revision of the Monster Manual provides for a creature a list of bonus languages. In such cases, those creatures are stuck picking bonus languages like PCs.
(The DM should, of course, use good judgment and common sense to pick a creature's extra languages. This DM recommends against these creatures all speaking Clockwork Horror, Druidic, Kython, or similar obscure or secret languages unless the DM has worked up an explanation—or, better yet, a plot!—for the common presence of oddball languages among his high-Intelligence-score, language-capable creatures.)
While this workaround may not technically adhere to the rules a reader can extrapolate from the most current Monster Manual, giving creatures that lack Bonus Languages entries extra languages due to their high Intelligence scores has the advantage of allowing the DM to have more creatures engage PCs in conversation, even if the conversations are—as I'd expect them to be with a slaughterking kython—abrupt, icky, and weirdly and creepily indirect. (I imagine kythons rudely talking to each other about the adventurers while the adventurers are, like, right there no matter what language the kythons are using.)
Finally, this means that if a PC's solar roommate swipes from the communal refrigerated portable hole a PC's clearly labeled leftover Kara-Tur food, the solar can't anymore claim (in Common, it says, due only to its tongues ability) that it's the PC's fault because—duh!—the PC labeled the food in Common, and the PC should know that the 3.5 revision means solars can only speak—and, therefore, can only read—Celestial, Draconic, and Infernal. Under these rules, that solar would know 6 extra languages, and if he didn't pick Common as one of them then how'd he read the ad for a roommate?
Note: The Wizards-of-the-Coast-licensed Rokugan for the Oriental Adventures product line includes a rule that sees some creatures that exhaust their bonus languages gain 1 extra skill point at level 1 per bonus language they can't take. For example, a creature that possesses Intelligence 14 yet has a bonus language list of but one language gains 1 extra skill point at level 1. While a little complicated and very niche, this reader has always respected that rule's author's foresight.