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.