If someone talks in an incomprehensibly way or in an unknown language, you might say that they’re talking double Dutch, nonsense, gibberish or rubbish. The reference to double Dutch dates back to the 17th century, when the English had a low opinion of the Dutch and their language due to the hostilities between England and the Netherlands, according to this site. A term related to double Dutch is apparently used in the world of IT is reverse Polish. Another way of indicating that something is incomprehensible is to say that it’s all Greek or Chinese to you.
The Dutch equivalents of talking double Dutch are Dat is Chinees voor mij (That’s Chinese for me) and Dat is Latijn voor mij (That’s Latin for me).
In Italian incomprehensible speech is compared to talking Ostro-Goth (parlare ostrogoto) or that it’s Arabic to you (per me è arabo).
The Czechs compare unintelligible language to a Spanish village – Je to pro mne španĕlská vesnice ((It’s a Spanish village to me).
In French they say such language is like Chinese, Hebrew or Javanese – C’est du chinois (It’s Chinese), C’est de l’hébreu pour moi (It’s Hebrew to me), Pour moi, c’est du javanais (For me, it’s Javanese).
In German they compare such language to Chinese – Spreche ich etwa chinesisch? (Am I speaking Chinese?), Bohemian villages (Böhmische Dörfer), or Chinese – Das ist Chinesisch für mich (That’s all Chinese to me).
Turkish speakers think of incomprehensible language as French – Olaya fransız kaldım (I am French to the conversation/chat).