In fact you wouldn't need to do anything like this in Studio 2021, where the language limit is gone.

You need to contact support: gateway.sdl.com/.../communityknowledge

Apparently I cannot even log in a support case - the link given in the article threw me back into the community. 
Nice to hear they solved the problem in Studio 2021, but I won't buy it, till they finally fix the 19th century-like interface and make it 64 bit.

Georgy Shcherbakov

How to reset Freelance languages for Studio 2017/2019. 

• Close Studio.
• Hold down the windows key and press R. (or right click the start/windows button and select Run).
• For Studio 2019 paste in this:
  reg delete HKEY_CURRENT_USER\Software\Microsoft\LDSRClient15 /f
• Press ok.
• Restart Studio.

Georgy Shcherbakov said:

I won't buy it, till they finally fix the 19th century-like interface and make it 64 bit.

What does 64-bit have to do with the interface?

Thanks a lot, Paul, you're a life saver!!! 
As for 64-bit version & interface, I think these two things are sorta related and are due to the fact that Trados requires modernisation badly and instead we get piecemeal improvements which I don't believe I'm ready to finance by buying an upgrade every 2 years. I used to do it, I upgraded 2017 to 2019, but after a careful consideration I decided upgrading it further isn't worth it until they modernise it properly.
Sorry for being a little bit brusque...

Georgy Shcherbakov

Georgy Shcherbakov said:

I think these two things are sorta related

In terms of the change from Winforms to WPF for the product then there is a very weak link in that we probably won't move to 64.but until the whole product has been upgraded to WPF.  But the main reason for this is related to the UI and has nothing to do with 64-bit at all.  It's more about screen resolution and the need to rewrite the product to be able to properly support higher resolutions.  This is done piecemeal because it's not trivial and we have been upgrading different parts of Trados Studio to WPF whenever we do something in this part of the software.

On 64-bit itself... moving to 64-bit is not going to improve the UI.

My personal opinion is that if this is your only reason for not upgrading then you're losing out, but of course this is something you'll need to come to terms with and make the appropriate decision.

Georgy Shcherbakov

Georgy Shcherbakov said:

I think these two things are sorta related

In terms of the change from Winforms to WPF for the product then there is a very weak link in that we probably won't move to 64.but until the whole product has been upgraded to WPF.  But the main reason for this is related to the UI and has nothing to do with 64-bit at all.  It's more about screen resolution and the need to rewrite the product to be able to properly support higher resolutions.  This is done piecemeal because it's not trivial and we have been upgrading different parts of Trados Studio to WPF whenever we do something in this part of the software.

On 64-bit itself... moving to 64-bit is not going to improve the UI.

My personal opinion is that if this is your only reason for not upgrading then you're losing out, but of course this is something you'll need to come to terms with and make the appropriate decision.