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

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

The first letter is only for internal purposes so that the software can calculate the percentage of translation.
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. 

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.