I'll say first I am VERY new to using the *.lang file and have historically done the evil deed of putting my string text in my code instead. 

However so far I can't find a way to set the tooltips of my GuiScreen / ConfigElement category buttons any other way other than the en_us.lang file.

So the good news is I can set the tooltip values of my mod config buttons now via my en_us.lang file ... but the bad news is that all the tooltips append the string ".tooltip" to the end of all of them - as seen here (or attached):

https://imgur.com/lmdLATO

 

Contents of my en_us.lang file

#PARSE_ESCAPES
gui.config.category.toggles=General Mod Settings
gui.config.category.values=Adjust settings for Humanoid and Related Mobs
gui.config.category.valuesla=Adjust settings for Large Animals
gui.config.category.valuesma=Adjust settings for Medium Animals
gui.config.category.valuessa=Adjust settings for Small Animals

 

Answering my own (noob) question in case anyone else out there gets stuck like I was. 

 

Basically I got it working by changing my I18n calls to this

I18n.format("gui.config.category.toggles")
I18n.format("gui.config.category.values")

And changed my en_us.lang file to this:

gui.config.category.toggles.tooltip=Toggles Tooltip text here
gui.config.category.values.tooltip=Values Tooltip text here

 

I kind of get the logic now ...