The bug

The information popup shown when signing into the Launcher using a Mojang account and clicking the  icon next to the email field is missing a comma.

As far as I know, there should always be a comma following a conjunctive adverb that starts a single independent clause. The last sentence should read:

Otherwise, use your username. 

How to reproduce

1. Select 'Mojang Account' when signing into the Launcher
2. Tap the  icon next to 'Mojang Account (email)'
   Observe the missing comma

Notes

This also affects 2.2.x beta versions.

According to Crowdin, this string can be found at:

apps/launcher/src/components/LoginPage/components/LoginForm/UsernameText.tsx:15