If you are using an exact quote from a participant, you should use an accurate quote, including any slang they speak. If the slang is too hard to understand, maybe that if not the best phrase to quote. Or you might have to include an explanation of what the slang word means [in brackets like this].
Yes, I agree with Michael W. Marek - if necessary use 'slang' word [explanation or appropriate word e.g., bostin' [amazing, brilliant or excellent] OR bostin' [Black Country slang for amazing, brilliant or excellent]
It would, however, depend on whether your institution style guide wants transcriptions to be basic or verbatim. So check with your institution's or department's research office for their style guide.
This may also help: https://www.transcribe.com/transcription-jobs-online/style-guide/basic-verbatim/