Clarify how to enable a new language on our translation platform

Originally created by @intrigeri on #16979 (Redmine)

• “You need to run this script with argument `—modify`”: I’m confused. Previously I’m instructed to run that script without any argument. If I have to pass --modify, let’s just add it to the copy’n’pasteable command line and drop this sentence?
• “You need to […] to actually create the missing po files and commit those files”: ambiguous English grammar, this could mean either that the script with --modify will add+commit the files, or that I have to do it myself once the script has run.

I’m happy to rephrase the doc (or let u do it) once you’ve clarified what you meant :)

Parent Task: #15086 (closed)