You can take lang/strings_english.txt and translate it to your language. Once you do that, you can attach it to this issue and we will include it into Mantis.

Thank you for taking the time to report a problem with mantis.

Since this problem report was originally made, a number of releases have occurred.
It appears that this issue has either been fixed, or may not be a relevant report for the current release.

Unfortunately you are not using the latest version and the problem might already be fixed. Please download the latest release from http://www.mantisbt.org/download.php

If you are able to reproduce this bug in the current release, or have some more information on how this feature could be improved in the current release. Please either change the mantis version on this bug report
to the version you tested and change the status back to "Open", or open a new issue report wit more information.

Again, thank you for your bug report and your continued support.
Paul

Esperanto strings were added on 2019-05-17 via TranslateWiki (see MantisBT master 62a5c91f ), but the file was named incorrectly (strings_eo.txt instead of strings_esperanto.txt), and furthermore was never referenced in $g_language_choices_arr so it could not actually be used in the system.

I contacted TranslateWiki team over IRC, and they fixed the file name on 2021-06-01 ( MantisBT master a91c415a ).