If police ranks are anything like military ranks, this is what they would be (and, in this case, "teniente" would be "First Lieutenant").

In some contexts, especially historical military contexts, "ensign" is just right for "alférez", but I wouldn't recommend it here. It's generally an old-fashioned term. It was replaced by second lieutenant in the UK and US armies in the nineteenth century. It survives in the US Navy, but it sounds wrong for a police rank. More importantly, as Jairo has pointed out, an ensign is an officer, albeit a junior one, but an alférez in the Colombian police is not.

And of course, as you say in the question, "second lieutenant" for alférez, which would be correct in army contexts, will create confusion with subteniente. I don't think you can use both "sub-lieutenant" and "second lieutenant" as different ranks; it's not immediately clear which is superior, and usually one or the other is used. And we still have the problem that a sub-lieutenant or second lieutenant is an officer but a Colombian police alférez is not.

The crucial point is to establish what a Colombian police alférez is in functional terms. It is essentially a senior trainee officer, above a cadete. It belongs to the category of "Alumnos":

"En la Policía Nacional, el cadete alcanza el grado de Alférez transcurridos 2 años de su proceso de formación para alumnos bachilleres y pasados 7 meses para los alumnos profesionales, dicha distinción posee unos símbolos y cada uno de ellos representa una cualidad del futuro oficial de Policía como lo son: el tiro, la cucarda, el sable y las botas altas."
https://es.wikipedia.org/wiki/Alférez#Colombia

"Oficiales superiores (Mando Directivo Superior)
Coronel (Cinco (5) años)
Teniente Coronel (Cinco (5) años)
Mayor (Cinco (5) años)
Oficiales subalternos (Mando Directivo Subalterno)
Capitán (Cinco (4) años)
Teniente (Cuatro (4) años)
Subteniente (Cuatro (4) años)
Profesionales en proceso de formación para Oficiales
Alférez (Seis (6) meses)
Cadete (Seis (6) meses)
Técnicos, tecnológos y bachilleres en proceso de formación para Oficiales
Alférez (Un (1) año)
Cadete (Dos (2) años)"
https://es.wikipedia.org/wiki/Anexo:Grados_de_la_Policía_Nac...

So it's a kind of senior cadet, someone who will become an officer (with the rank of subteniente) but hasn't become one yet. I think the category of "officer candidate" fits this, with "officer designate" as an alternative. The former term is used in the US. The latter exists in some places and is certainly accurate.

"Officer candidate or Officer aspirant (OA) is a rank in some militaries of the world that is an appointed position while a person is in training to become an officer. [...]
In several NATO countries, the term Officer designate (OF-D) is used. In the NATO rank scale, it comes below the grade of OF-1 and above the grade of Student Officer. [...]
In the United States Army, officer candidates attend either the Federal Officer Candidate School (OCS) at Fort Benning, Georgia, Federal military academies, or ROTC programs at a civilian university."
https://en.wikipedia.org/wiki/Officer_candidate

And for subteniente, you could use Sub-lieutenant, though I would recommend Second lieutenant, which in NATO armies is usually the rank immediately above cadet (there's no real equivalent to the Colombian alférez). Above Second lieutenant comes Lieutenant or First lieutenant (Colombia's teniente) and then Captain (Colombia's capitán).
https://en.wikipedia.org/wiki/Ranks_and_insignia_of_NATO