What is ;T;N or ;T;T for in the language file?

By default all of the rows in the language file end with:
;N;N

The first letter is only for internal purposes so that the software can calculate the percentage of translation.
E.g.:
if there are 3000 rows in the language file, and 300 of them are ending with ;T;N  then the software based on that will determine the translation to be done for 10%.   (300 / 3000 * 100)

The second letter means, whether it is a 'custom' translation or not. More exactly, if there is any row ending with:
;N;T (or ;T;T)

then that last letter (T) tells the software, that it is a custom translation and should not be modified by any further update of the software (it should be left intact). 


None of those endings have any impact on the fact, whether a translation is populated or not. 

NOTE:
When we talk about 'Contact Forms', more exactly about the 'field names' of a contact form, then the field names are not included in the language translation file. A customer is supposed to create a unique contact form for each language they provide support in   and  name the form field names accordingly.