For the record: I am no expert in the japanese language and am thus tinkering around with this, with that said however, the Japanese language in eSpeak-NG with any of the klatt voices (and the announcer voice) will produce weirdly sounding artifacts on certain phonemes, particularly with [[v]], [[n]], [[N]] and [[m]].

The best way to describe the glitch in the klatt synthesizer would be that it is too nasal with those phonemes whereas in the default voice it is not the case, the rest seem fine though, however with [[N]] the higher formants are loud, when that is not the case with the default voice.

klatt_glitch.zip

Attached is a zip archive containing two wav files which showcase this glitch in the japanese language, generated with [[vv]]. [[nn]]. [[NN]]. [[m]].