When you try to translate the “Form Titles for Purchasing Document Printouts”, which is Table T166U, “Translation” is grey in the customizing: 
The entries can be translated using transaction SE63. In SE63, go to Translation -> ABAP Objects -> Short Text:
On the popup screen select A6 Table Entries -> TADG Tables (Customizing – No SAP UPD, Only INS):
On the next screen, enter the source and target language for translation and search for “*T166U*: 
On the popup screen click continue:
The Object Name is filled. Click on “Edit”:
On the next screen, select the entries you want to translate:
On the next screen, you can enter the translations:
In order to transport your translations, you can use transaction SLXT.