The use of 'in' is to indicate the language, the 'medium', used in a particular situation. It suggests a contrast, a distinction from other possible languages. So, a conference held in Moscow with delegates from all over the world, may be held in English (as opposed to Russian even though it is held in Moscow). Delegates will be speaking in English when giving papers.
When chatting among themselves, delegates might be speaking English, Russian, German etc.
The value of the Rosetta Stone in deciphering hieroglyphic writing is that the carved text is made up of three translations of a single passage: two in Egyptian language scripts (hieroglyphic and Demotic) and one in classical Greek.
A German tourist in London may be speaking in English (since almost nobody would understand him if he spoke German), but when alone, his thinking is probably in German.
We can similarly say that I, an Englishman, wrote a letter in German to my pen pal in Berlin (as opposed to my native English).
One of the oddest experiences I had was being in Egypt, in a small village near the Suez Canal, trying to ask at the small train station about the next train. I don't speak Arabic, and he didn't speak English. He then spoke in German, in which I can get by, so the conversation was in German - we both began to speak German.
When it comes to 'read', we can say:
Yes, I have read the English translation of Faust, and also read it in the original German.
Thank you so much, David. A student of mine asked me this question, and simple though it looked, I couldn't say anything reasonable to explain that fluctuation.Now I'll know what to say. You've saved my good name.