Language house rules I have used:
In a real-world version of D&D I ran, outsider languages sounded like instrumental music of different genres. The principle was that outsider speech was not encumbered by mere phonemes or even intonation but was more complex and information-dense than any humanoid tongue.
Celestial: classical music on pipe organs
Abyssal & Infernal: Heavy metal on electric guitars. Different fiendish races had varying "accents" corresponding to different styles of heavy metal: death metal, thrash metal, black metal, etc.
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I have also allowed many languages to grant a +2 circumstance bonus to certain skill checks when the language is spoken or written as part of the skill check. For some languages, the bonus only applies when using it to instruct or explain something to someone else, whether you are using the subject language itself to communicate or whether you are communicating in the speaker's or reader's native language (which can include Common). Either way, fluency in the associated language gives you the ability to better understand the topic at hand and to be more effective at it. Because all these modifiers are of the same type, bonuses granted by different languages to the same skill do not stack.
For example: A character who knows Elven and Orcish can rely on his linguistic experience to either (a) improve his Diplomacy check and make friends with the NPC, or else (b) better Intimidate them into submission.
Originally I had only worked out the bonuses for Elven, Orcish, and the outsider languages, because my players never really found a need to use anything else. But these house rules could be expanded to include almost every language in the core rules. There is certainly a lot of overlap, but maybe these house rules will give your players a reason to take Aklo or Sylvan or even Giant, beyond the unlikely contingency of being able to talk down a hostile monster.
Abyssal, Celestial, and Infernal: +2 to Knowledge/planes and Knowledge/Religion checks.
Aklo: +2 to Knowledge/dungeoneering and Knowledge/planes checks.
Aquan, Auran, Ignan, and Terran: +2 to Knowledge/planes checks. Though it might make sense to do so, I don't feel the need to confine the usefulness of an elemental language to the elemental planes themselves, to its plane of origin, or even restrict its usefulness to non-opposing elemental planes.
Draconic: +2 to Knowledge/Arcana and Use Magic Device checks. Curiously, this even applies to Kobolds...
Druidic: +2 to Knowledge/Nature and Handle Animal checks. Though a secret language, it is well adapted to the agenda and lifestyle of its users.
Dwarven: +2 to Craft, Appraise, and Knowledge/Engineering checks. Dwarves are master craftsmen and have had to develop many technical and scientific terms for all kinds of professions, from mining to gemcutting to blacksmithing.
Elven: +2 to Diplomacy and Sense Motive checks. Elven vocabulary is quite advanced in its grammar and verb tenses, yet very subtle, soothing, and romantic. The language itself is fairly immune to change relative to other languages, due to the longevity of its native speakers. An Elven document written twenty thousand years ago is no more difficult to understand than one written yesterday, which makes the language the ideal medium for treaties and other legal documents. Sense Motive is also included because the well-defined nuance of Elven vocabulary is so precise, any attempt to doublespeak in Elven is easily detectable.
Giant: +2 to Knowledge/geography checks and Knowledge/dungeoneering checks. Giants get around a lot.
Gnoll: +2 to Survival checks.
Gnomish: +2 to all verbal Perform skills (Acting, Comedy, Oratory, and Singing) and +2 to Bluff checks when attempting to deceive the listener or reader or when attempting to pass along a secret message. Verbal artistry is in the blood of every gnome, as is the ability to entertain and confuse their audience (and occasionally both at the same time!) There are few languages whose words carry so many double meanings and vague, almost contradictory connotations.
Goblin: +2 to Intimidate (since it has a lot in common with Orcish) and Knowledge/dungeoneering checks. Although, what could a Goblin hope to Intimidate?
Orcish: +2 to Intimidate checks. The language is filled with all kinds of cutting innuendo and harsh insults, and has all kinds of visceral synonyms for words that have to do with combat, warfare, and torture. It is the extremely rare orc who knows more than half of them, but all dialects taken together orcish is no less hobbled for vocabulary than any other natural tongue.
Sylvan: +2 to Knowledge/nature, Knowledge/geography, and Survival checks.
Undercommon: +2 to Knowledge/arcana and Knowledge/dungeoneering checks.
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