Why elasticsearch used different languages like painless and groovy in different cases?

[saikrishnagaddipati (krishna_gaddipati)]

As we know Scripting is supported in many APIs. Can someone explain me at which scenario we should use groovy and which scenario we should use painless or other scripting languages?

I was going through the elastic documentation and it says to use groovy in this link

Partial Updates to Documents | Elasticsearch: The Definitive Guide [2.x] | Elastic

and in other places of the documentation i find documentation referring to painless scripting.

I was trying to see if i can use single language in my whole elastic environment?

[Christian_Dahlqvist]

As the use of Groovy within Elasticsearch is being deprecated, you should always use Painless.

[rjernst (Ryan Ernst)]

Groovy isn't just deprecated, it was completely removed in 6.0.