Building a contact form from scratch
Here are 2 possible paths. I used the first one:

Path 1:

Stage A:

Go to Structure > Contact forms, and there to "Personal contact form".


Under "Manage fields" delete any custom fields you might have added.
Under "Manage display", make everything as hidden (under both labels &amp;&amp; formats).
Optional - Under "Manage form display", make everything hidden.


This will totally hide the native personal contact form.

Stage B:

Create a new form, from scratch; Add desired fields to it under "Manage fields", and under "Manage form display" &amp;&amp; "Manage display" hide all automatic fields taken from "Personal contact form".

Link to the new form from a menu, and that's it.

If you are new to Drupal in general and using Drupal 8, check Youtube for video tutorials on how to create basic contact forms and make sure to use the steps above if you want to build your contact-form from scratch.

Path 2

It is best to use the native "Personal contact form" and its native fields that are auto-replicated for other secondary forms if you pursue a minimalist approach, and I, as one who pursue such approach would gladly use it if I could have its fields auto-translated to my sites default language, but without the well known bug in the current Drupal 8 branch (8.1.x) yet to be fixed:

Due to the bug, you can't use the management interface in English and forced to have a XYZ language translated management interface - In some cases it's problematic from at least one reason).

Until this bug will be fixed I personally will use the first path, and might change to path B sometime in the future (in any case its a matter of minutes). 

Notes

I suggest creating your block names only as a singular-phrase names, that doesn't contain any non-letter characters.