Monese

Monese is the name given to the language spoken by Pokémon. It is not normally able to be understood by humans, but anyone who has used the Human-Pokémon armband can understand it, and it can be learned just like any human language.

Known methods for humans to understand Monese include:

• Being pretty much raised by Pokémon from a young age. (N and Fool).
• Using a Psychic type to translate. (Tagg before his trip to PMD-A.)
• Via a Glitch (Wolf).
• Visiting the Dream World (Crewe).
• Being transformed into a Pokémon either through a trip to the PMD universe (Tagg) or by wearing the human to Pokémon armband (Rex).
• Being formally taught the language (Umbramatic).
• Having a Pokémon directly translate through electronic methods (Ren)

While all mons seem to speak the same Monese "language", which appears to be directly genetically encoded since transformed humans keep the ability to understand it upon changing back, dialects and (sub?)languages are known to exist:

• Dialects of real world languages exist.
  • Members of some Murkrow Mafias speak in "Russian Monese".
  • There also exists Chinese, Japanese, Spanish, Italian, French, and German.
• Kanto and Johto Nidoran employ Lapine from Watership Down in formal situations. (Information available at Bits'n'Bob-stones)
• Dragon mons have a Draconic dialect based on Skyrim as well. (Information available at http://thuum.org/ )
• Some Pokémon also speak in Al Bhed from Final Fantasy Ⅹ. (Information available at Al Bhed (Final Fantasy Wiki))