but the language string is bug_history

I don't see any error.
Isn't there $s_jump_to_history = ... in your language file?

I assume you are using Dutch language settings, where no one translated this string (introduced in version 2.6.0) until now.

You can fix it yourself.
Translations are managed via TranslateWiki. Please refer our own Wiki [1] for the process to submit new translations or revise existing ones.

[1] http://www.mantisbt.org/wiki/doku.php/mantisbt:translationshowto

I use the dutch language but there isn't a jump_to_history string. If I check the german language file there is, so it's missing in the dutch file.
Also here on this website, on top I see jump to history.

jumptohistory.PNG (1,953 bytes)   

jumptohistory.PNG (1,953 bytes)   

I use the dutch language but there isn't a jump_to_history string.

That's because of what I told before:

no one translated this string (introduced in version 2.6.0) until now

Translations are managed via TranslateWiki. Please refer our own Wiki [1] for the process to submit new translations or revise existing ones.
A few days after you have submitted the new translation, it will become part of the official Mantis source code and will become part of the next released version.

[1] http://www.mantisbt.org/wiki/doku.php/mantisbt:translationshowto

Will be part of 2.13.0 https://github.com/mantisbt/mantisbt/blob/master/lang/strings_dutch.txt#L88