There are two issues with the message.

Firstly it uses concatenation to build a sentence. It is a no go, different languages have different word order and what works in English might force people to make a very awkward sentence in their language (for example Japanese or Turkish have verbs at the end of the sentences). See also https://www.mediawiki.org/wiki/Localisation#Avoid_fragmented_or_'patchwork'_messages

Secondly, it asks about MediaWiki account to log in to Wikimedia Commons. I assume you mean MediaWiki.org account, using just the wiki engine name is confusing, IMHO, but to be honest have you considered routing OAuth through Wikimedia Commons instead? It would be the least confusing, IMHO (but this goes beyond just fixing the message in such case).

URL: https://translatewiki.net/wiki/Wikimedia:Qrcode-generator-pleaselogin/uk