What happens?: {{#language:sr-el|en}} returns 'Serbian (Latin script)'
Taken at face value as an IETF language tag, sr-el is invalid because el is not an assigned ISO 3166 region subtag.

Of the twenty-one languages that include the disambiguation (Latin script), all except sr-el use the ISO 15924 script subtag -Latn to denote Cyrillic script (kk-Latn, ku-Latn, tg-Latn, tt-Latn, ug-Latn, uz-Latn, aeb-Latn, bbc-Latn, cja-Latn, cjm-Latn, crh-Latn, gom-Latn, hif-Latn, ike-Latn, osa-Latn, phn-Latn, ruq-Latn, shi-Latn, shy-Latn, and tlh-Latn)
What should have happened instead?:
sr-el should be changed to sr-Latn