This was implemented long ago by shs, and then reverted. I couldn't find the commit right away, I'm 95% sure it was before we used github.
The problem isn't deleting it, it is what happens on a multi-language board.
For example:
Re: Enabling to translate "Re:" on others languages [English user replies]
От: Enabling to translate "Re:" on others languages [Russian user replies]
Ré: Enabling to translate "Re:" on others languages [French user replies]
От: Enabling to translate "Re:" on others languages [Russian user replies]
回复:Enabling to translate "Re:" on others languages [Chinese (simplified) user replies]
Re: Enabling to translate "Re:" on others languages [English user replies]
Etc.
This would be OK if you, presumably like most boards, only had one language pack installed. It was simply decided way back in history that leaving it in "Latin" was preferable to showing potentially dozens of prefixes.
EDIT: Actually, it's worse than this, as it was implemented long ago. Taking the same example I used earlier, what that implementation did was:
Re: Enabling to translate "Re:" on others languages [English user replies]
От: Re: Enabling to translate "Re:" on others languages [Russian user replies]
Ré: От: Re: Enabling to translate "Re:" on others languages [French user replies]
От: Ré: От: Re: Enabling to translate "Re:" on others languages [Russian user replies]
回复: От: Ré: От: Re: Enabling to translate "Re:" on others languages [Chinese (simplified) user replies]
Re: 回复: От: Ré: От: Re: Enabling to translate "Re:" on others languages [English user replies]
And pretty quickly you'd hit the 60 character post subject limit.
Relevant commits:
https://github.com/phpbb/phpbb/commit/5 ... f12fa83049
https://github.com/phpbb/phpbb/commit/a ... 557ff36e2f