This issue was discovered following resolution of 0028648.

Follows discussion in #translatewiki IRC channel

==== 01-06-2021 ====
10:42:48 — @dregad: Greetings, I have a question regarding serbo-croatian language (sh)
10:43:05 — @dregad: Apparently this code is "deprecated" https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
10:43:55 — @dregad: Not really sure what that means, or what are the implications
10:45:41 — @dregad: So it is now a "macro language"
10:45:59 — @dregad: Is it still valid and maintained in Translatewiki ?
10:48:46 — @Nikerabbit: dregad: it's enabled yes
10:49:00 — @Nikerabbit: but it seems inactive
10:51:41 — @dregad: I am asking, because following the new hindi translations for MantisBT you fixed yesterday, I checked for other occurrences of languages for which there are translations, that are not configured in Mantis code
10:51:53 — @dregad: And I found sh and eo
10:53:32 — @dregad: So I checked the ISO codes to know what they were and found out about the sh thing - the language was added in 2019, and there's been a few updates since then
10:55:02 — @dregad: If it's active on Translatewiki, I guess we can ignore the ISO deprecated status and use it
10:55:59 — @dregad: Nikerabbit I would need you to rename the files though, as you did for hindi
11:15:40 — @Nikerabbit: I can ask my colleaugue if they have more insight into sh
11:43:06 — @dregad: Nikerabbit: cheers ! For sh, depending on your colleague's feedback of course, it should be
11:43:10 — @dregad: strings_sh.txt => strings_serbo_croatian.txt

==== 02-06-2021 ====
13:13:15 — @dregad: Nikerabbit: did you hear anything about sh language?
13:14:10 — @Nikerabbit: dregad: I'm considering to disable it for all projects except for mediawiki
13:14:55 — @Nikerabbit: now afk for a couple of hours

==== 07-06-2021 ====
10:33:43 — @dregad: > <@Nikerabbit:libera.chat> dregad: I'm considering to disable it for all projects except for mediawiki

Nikerabbit: did you reach a decision on disabling the sh language ? I'd like to know if I need to fix the invalid filename 'strings_sh.txt' in MantisBT (it should be 'strings_serbo-croatian.txt') or just ignore it.
11:01:24 — @Nikerabbit: dregad: ha sorry for not being very proactive on this thing, I am training last and this week with little bandwidth for other things. My suggestion would be to just ignore it and I'll look into disabling in the future.
11:03:02 — @dregad: OK. No worries and thanks for the feedback