I thought this was funny and sad at the same time

[Alfonso]

Poli, se diría: El ladrón habló a su víctima en un tono amenazante.

I don't think the Spanish registro translates the English tone.

In Spanish you can distinguish among registro, tono y estilo. Registro is the most technical word of three of them. You don't use it in everyday conversation, but it's helpful when learning a language, because any speaker uses different registros depending on the context.

Registro differs from tono, among other points, in the fact that it can gather a number of stereotyped ways of speaking.

Diferentes registros (I don't put it into a normal conversation as it is a technical word):

When speaking to your grand mum:¿Qué pasa, abu, no vas a usar el coche este finde, verdad?

Your layer, when speaking to a jury:Con la venia, señoría, mi patrocinado se declara inocente de los cargos que se le imputan, y solicita el sobreseimiento de la causa.

When speaking to your jail mate:Sí, tío, ese abogado estaba borracho. Qué cacho hijo de...

Again speaking to your grand mum:Lo siento, yaya, cuando salga te juro que pago todos los daños del coche.

Anyway, Iris gave a very good example of the usage of registro: Official documents are written in a formal register.
This can be translated into Spanish as: Los documentos oficiales se escriben en un registro formal.

So, I understand that in English you can say register for the Spanish registro. But what would you say for registro de habla? Talking register, speaking register...?

[Iris]

You say "to give an example"

[Rusty]

My two cents:

Registro seems to translate into manner, or way of speaking - as in, "He spoke in a demeaning way," or "She spoke in a condescending manner." If we want to be understood among certain groups, we sometimes need to adopt their way (manner) of speaking.

Here is the definition from the RAE:
Modo de expresarse que se adopta en función de las circunstancias.

[Tomisimo]

I'll throw in my $0.02.

As far as I know, from the few linguistics classes I took in college, register (registro) is a specific linguistics term used to refer to the type of language you use based on the situation you're in.

When changing register, many different things can change: vocabulary, pronunciation, intonation etc.

IIRC, by convention, register is specified on a scale from 1 to 5, 1 being the most informal and 5 by the most formal.

Register 1: Hey wassup? (two college students [friends] together)
Register 2: Hi, how's it going? (customer greeting the person taking their order in a fast food restaurant)
Register 3: Hi, how are you? (college student greeting their professor)
Register 4: Good afternoon sir, how are you doing today? (a waiter greeting a customer at an expensive restaurant)
Register 5: *** some pompous way of saying hello to a dignitary or the queen or someone. :)

These are basically five different ways of saying the exact same thing, but social norms cause you to express the same thing in different ways, depending on the situation you're and who you're talking with-- or in the case of writing-- who your audience is.

Another example:

Register 1 or 2: The car crashed into the phone pole.
Register 3 or 4: The vehicle collided with the telephone pole.

[Alfonso]

Thanks a lot, David, your explanation is really clear and works as well for the Spanish registro, that, I guess, can be translated into register.

Now, we have the task to explain tone / tono.

Another question: What you call registro de habla? Speaking register or something?