this came up last night.  1 side said since it mentions training in med/hvy armor you need at least an armor proficiency.  Other side said it grants you the proficiency for this set of armour. 

You gain a +1 bonus to Armor Class while you wear this armor. You are considered trained with this armor even if you lack training with Medium or Heavy armor.

You do not need any armor proficiency it mentions medium and heavy because chain shirts are medium, chain mail is heavy armor. So depending on which one you possess determines which proficiency you do not need as you are considered trained with that armor. You don't need light, or any armor proficiencies. I am not even sure where anyone gets the idea you need a proficiency of any kind. 

Quote from jasperrdm >>

this came up last night.  1 side said since it mentions training in med/hvy armor you need at least an armor proficiency.  Other side said it grants you the proficiency for this set of armour. 

You gain a +1 bonus to Armor Class while you wear this armor. You are considered trained with this armor even if you lack training with Medium or Heavy armor.

 The Elven Chain doesn't grant any armor proficiency. Instead, while you wear this armor, you are considered trained with it if you don't have Medium or Heavy armor training already.

So for exemple a Wizard can wear an Elven Chain — mail or shirt — and not have the drawbacks  ''Disadvantage on any D20 Test that involves Strength or Dexterity, and you can’t cast spells.''

How can you not have disadvantage but still be unable to cast spells?

Mydude is right, by RAW.

Anyone can put on any suit of armor.

If you aren't trained you take a penalty.

Elven Chain gives you the training.

You don't take the penalty.

To be nitpicky about it, there technically is no such thing as Armor Proficiency in the game, just Armor Training. Proficient only applies to weapons, skills and saving throws.

Quote from El_Jefe >>

To be nitpicky about it, there technically is no such thing as Armor Proficiency in the game, just Armor Training. Proficient only applies to weapons, skills and saving throws.

 thanks. This was a major major help.

I'd say a spellcaster can cast spells while wearing the Elven Chain:

You must have training with any armor you are wearing to cast spells while wearing it. You are otherwise too hampered by the armor for spellcasting.

I honestly think that was just a typo by Plaguescarred. I'd say you can cast spells as well.

Quote from jasperrdm >>

Quote from El_Jefe >>

To be nitpicky about it, there technically is no such thing as Armor Proficiency in the game, just Armor Training. Proficient only applies to weapons, skills and saving throws.

 thanks. This was a major major help.

In fairness, it was called "armor proficiency" right up until 2024. A lot of people still use that term, even though the latest round of core books don't anymore, which is probably where the confusion came in.

Quote from Plaguescarred >>

 The Elven Chain doesn't grant any armor proficiency. Instead, while you wear this armor, you are considered trained with it if you don't have Medium or Heavy armor proficiency already.

So for exemple a Wizard can wear an Elven Chain — mail or shirt — and not have Disadvantage on any D20 Test that involves Strength or Dexterity, and you can’t cast spells.

In previous editions, being trained in armor and being able cast spells in that armor were separate abilities. In 5e, if you are trained in an armor, you can cast spells in that armor.

Exactly so spellcasters can cast spells in Elven Chain just like they can do so while wearring armor they're trained with.

Armor Training allows you to use armor of a certain category without the following drawbacks, which this magic item let you be considered as such.

Quote from El_Jefe >>

To be nitpicky about it, there technically is no such thing as Armor Proficiency in the game, just Armor Training. Proficient only applies to weapons, skills and saving throws.

Proficiency doesn't only applies to this but Tools as well.

Quote from Plaguescarred >>

Exactly so spellcasters can cast spells in Elven Chain just like they can do so while wearring armor they're trained with.

Armor Training allows you to use armor of a certain category without the following drawbacks, which this magic item let you be considered as such.

You might want to edit post 4. Several of these comments are just from you accidentally writing "can't" instead of "can".

Quote from sabin76 >>

Quote from Plaguescarred >>

Exactly so spellcasters can cast spells in Elven Chain just like they can do so while wearring armor they're trained with.

Armor Training allows you to use armor of a certain category without the following drawbacks, which this magic item let you be considered as such.

 

You might want to edit post 4. Several of these comments are just from you accidentally writing "can't" instead of "can".

 I'm saying that you do not have ....

But i will edit to clarify better.

Quote from sabin76 >>

I honestly think that was just a typo by Plaguescarred. I'd say you can cast spells as well.

 I know, I know. I'm seeing now that he indeed updated his reply, but tbh, my answer was just a quick opinion after reading Taenx's question in reply #5.

I was on mobile, so I didn't read Plaguescarred's answer or the previous ones in full detail, but I'm confident he addressed the OP's question well :)

EDIT: for clarity.

What i originally said had no typo, i copied drawbacks you don't have. 

But ended up editing my post for more clarity.

5E24 Elven Chain is TEMU version made of fake mithril. Order 2-3 size larger than normal, and don't take it into Antimagic Field or else it no longer work plus no refund exchange only shipping back oversea to Evermeet takes forever ....  😊

Quote from wagnarokkr >>

Quote from jasperrdm >>

Quote from El_Jefe >>

To be nitpicky about it, there technically is no such thing as Armor Proficiency in the game, just Armor Training. Proficient only applies to weapons, skills and saving throws.

 thanks. This was a major major help.

In fairness, it was called "armor proficiency" right up until 2024. A lot of people still use that term, even though the latest round of core books don't anymore, which is probably where the confusion came in.

 Also the fact I started back in 1980 and have at least 4 editions of knowledge floating through out  me head.  I wonder why my players loved me the night, i allow them to start a lightning bolt where they wanted to as long as was in range.  The Fire giants were not happy with it.

Note for the heavy armor version you still need str = 13 or have speed reduced by 10, and it gives you disadvantage on stealth like regular chain mail.

Quote from Tigero >>

Note for the heavy armor version you still need str = 13 or have speed reduced by 10, and it gives you disadvantage on stealth like regular chain mail.

 Yes, it's more clearly noted on the Elven Chain Page at least.