If you ever had to do extra work by creating English strings/texts when your client/project doesn't even need English...
... or maybe you just didn't bother to wrap your strings with t() because you just need them for this site and you know that doing so would require them to be English, and will cost you more time and money...
... If you're sick of needing to retranslate everything every time an English string that you didn't need in the first place needs to be fixed...
... or you just, wtf, want to write the module's text in your own language, because you have better things to do in life than writing strings twice...

And overall, if you want to save your time -and/or your client's money- when coding for a project that doesn't need to be in English... still being able to translate the UI in the future to other languages (also English)...

Then you know what I'm talking about...

(and if you don't please move on, there are plenty of other interesting sessions by more talented speakers on this site)...

Come over here and we'll see how Drupal 8 will allow writing your modules in your own language... and translate them later without changing the code....

Tips:
• We'll be using -and abusing- Drupal 8 localization system
• ... And maybe the brand new configuration system too...
• Some people may tell you these are 'evil' ideas; I'd rather call them 'creative'...

Guaranteed:
• Your modules won't be accepted in Drupal contrib, there are enough modules there anyway ;-)
• You will save time and money when developing non English speaking web sites.