So first, a disclaimer: I'm less than beginner level in everything that has to do with programming. I'm analysing the occurrences of English code-switching/translanguaging in a particular Polish-language IM groupchat. I've already compiled the conversation into a single text document (like 700k words in total, I know, a lot.) and now I'd like to extract just occurrences of English words among all the Polish words. How do I do it? I'd be super grateful for any beginner level explanations. p.s. I managed to upload the data to sketchengine too.
It seems like you are about to make a little corpus analysis. You already gathered different discourses together and created a corpus. I would recommend to use https://www.laurenceanthony.net/software/antconc/ It is a free professional software which I still use when analysing my own corpora. It is not that difficult to use and analyses a lot of things.
First of all, you should list all the words that occur in your corpus. Just ignore all the words that are not in English. As the next step you should think about what you are really interested in. "...occurrences of English code-switching/translanguaging in a particular Polish-language IM groupchat." Think about a hypothesis and then go on with the actual analysis.
Now you should look at collocation and stuff like this. So look up the words you found in the program and look where exactly they occur. What is there purpose and why does the speaker use them at this specific point. In Antconc this is pretty easy. You simply type in the word you want to analyse under the concordance tab and it will show all the passages of your corpus where they occur. After this it is up to you what exactly you want to look at.
And by the way, code-switching and translanguaging are not synonyms for each other. They are concepts about completely different things. I believe that you mean code-switching where you switch inside of a conversation to another language. Translanguaging is something different and should not be fortaken for code-switching. Furthermore you are also not comparing corpora. You are simply analysing a corpus.