I know that like Star Wars' droid, Pikachu's language is just nonsense with emotionally appropriate intonation, that said...
Can suprasegmental features (accent, tone, etc) encode a single three syllable word in enough different ways to get the vocabulary up to toki pona size? (about 100-125) Or up to a natural language size lexicion (say 7000-15000)
The question occurred to me because Alexa has a "speak to pikachu" feature where it then pretends to let you converse with Pikachu in Pikachu-ese.
From what I remember from the cartoon, Pikachu could only say certain permutations of its name, with any combination of vowels elongated.
3 syllables pikat͡ʃu 8 words
2 syllables pika 4 words
1 syllables pi 2 words
Pikachu is also capable of producing at least 4 different tones:
Tone IPA Made when
High ˥ Excited
Mid ˧
Rising ˧˥ Curious
Falling ˧˩ Disappointed
So, knowing this, Ash's pikachu should be able to say at least 584 distinct "words", which is well above the toki pona threshold.
Sadly it's pretty much impossible for this system to match the lexicon size of a natural language. Using an 8 tone system and adding primary and secondary stress brings the number of distinct words up to 25104, which is significantly larger than the Esperanto lexicon but still far away from that of a natural language, which is around 200K words.
From here on you could introduce some questionable features like 8 more tones, 7 different vocal registers, or the ability to say different permutations of its name (which is probably not as dodgy but also imagining pikachu say /ka.chu.pi/ makes me extremely uncomfortable). The best way in my opinion would be to introduce the syllabic thunder shock ⚡, which would require pikachu to release up to three thunder shocks whilst saying a word, all for the good cause of bringing the lexicon size of Pika-ese to a final 198688 words.
(No wonder Meowth learnt how to speak a human language, imagine being stuck with a one-syllable name.)
There is such thing as 'whistled' languages which rely solely on tone. Though they're all based on spoken languages. All the 'speaker' does is mimic the tonal pattern without pronouncing any phonemes. Such languages are used by hunters to communicate without alerting their quarry, because the language sounds just like bird song. But they don't use this language for most day-to-day communication.
As for a pikachu, well their 'words' are made up of three possible syllables: pi, ka, and chu. Thus a pikachu would have a total of 3 possible one-syllable words, 9 two-syllable words, and 27 three-syllable words. As we see, words would need to get quite long to get the possible vocabulary up to 100. Using tones though would easily multiply this. Even a simple two-tone system would double the number of possible syllables. Thus the syllable count would be 6 one-syllable words, 36 two-syllable words, and 216 three-syllable words. And there are real languages that have as many as 6 tones. With such a system, a pikachu would have 18 possible syllables to work with. Of course, you could say pokemon are more sensitive to tone than humans, which could increase the number of tones beyond what any human could manage.
Though a problem arises with cross-species communication. Obviously, no two species have the same set of syllables. Thus, for different species to be able to communicate with each other, the poke-language would probably have to be purely tonal, like a whistled language. Though you could 'multiply' the number of tones using length. A tone held for a short time could be considered a separate 'phoneme' from the same tone held longer. But we might just be fooling ourselves. The pokemon may just see a long tone as two identical tones right next to each other.
Most pokemon of course aren't intelligent enough to warrant a full-fledged language. Even Toki Pona would like contain some words they wouldn't make use of. Yeah, there's things like Alakazam which have an IQ higher than humans, but of course they can communicate telepathically, which would completely remove their need to use a language. And of course, there's some that can learn human language. Team Rocket's Meowth not only can speak, but he even taught himself how to! There's also one of the early movies which features a talking slowking. And one episode of the anime has a tenacool possessing meowth and speaking through him, which would imply it knows and understands human language, but lacks the ability to actually produce human sounds. Though honestly, its astounding that any pokemon could pronounce a human language. We have a lot of features in our mouths unique to our species that allow us to talk. Some birds in the real world can mimic human speech, but they accomplish this differently from how we do. I believe there is a pokemon in one of the later generations that can mimic human speech, though it can't understand language. By the way, the franchise is quite inconsistent as to whether pokemon can understand spoken speech or not.
