https://www.w3.org/TR/WCAG21/#understandable
Word choice: use the simplest word that conveys your meaning. [1] Prefer the short word to the long. [2]
"Alternatively, you can scan the following QR code in Google Authenticator."
"Or, you can scan the QR code in Google Authenticator."
"The indexer is being initialised. Do not close this window."
The indexer is starting. Do not close this window."
https://www.plainlanguage.gov/guidelines/words/use-simple-words-phrases/
https://en.wikipedia.org/wiki/List_of_plain_English_words_and_phrases
[1] http://plainlanguagenetwork.org/plain-language/what-is-plain-language/ [2] https://www.plainlanguage.gov/guidelines/words/
You're on the right track!
GOOD: alternatively => or
GOOD: omitting the word "following" - I take it there's only one QR code being shown.
GOOD: is being initialized => is starting. Cringe, that was horrible.
Keep iterating. I'm not sure of the exact context, but maybe...
The indexer is starting. => Updating the index. (This is a good candidate for a symbol too, like a progress bar or spining wheel. Is there a "finished" message?)