When I look in my version of the Catalog translation file I find the following lines for the three themes
"Related Products","Related Products"
"We found other products you might like!","We found other products you might like!"
"More Choices:","More Choices:"
They are used in the
Catalog\templates\product\list\items.phtml template which is used to generate these types of list as widgets, and they are setup properly translatable in that template. The corresponding dictionary is in the Catalog module folder.
It's probably worth trying these.
But if you are not able to use inline translation on these, and you are seeing terms like "Up-sells" on the front end, then perhaps there is a third party module or a widget in the way, and you might have to look at their settings and templates. If the title is set up in the module's admin interface for example a "custom product tabs" style module
And if it is on the back end side that you need to translate, then you need a separate theme to translate these