Latin

Latin is the ancient language of the Romans, which Sheldon studied at school until 5th grade.

In The Electric Can Opener Fluctuation (S3E01), Sheldon uses Latin phrase " Post hoc, ergo propter hoc". In The Hamburger Postulate (S1E07), Sheldon uses the phrase "Reductio ad absurdum". In The Zazzy Substitution (S4E03), Sheldon uses the phrase "Ipso facto".

In The Jiminy Conjecture (S3E02), both Sheldon and Howard know the Latin names of insect species. Also, Sheldon either remembers particular Latin quotes or that he is able to translate short phrases into Latin.

Howard: The common field cricket, AKA Gryllus assimilis, which is Latin for "suck it, you lose". Sheldon: The snowy tree cricket. AKA, Oecanthus fultoni. Which is Latin for "I'll suck nothing". Of course I'm joking because the Latin for that is nihil exsorbibo.