In templates/classic/html/issue.search.html :

*: The "all text" field will look in message bodies and
issue titles<br>
 **: If you supply a name, the query will be saved off
and available as a link in the sidebar

cannot be translated.

The reason is that in .po files, the msgid string is :

"*: The \"all text\" field will look in message bodies
and issue titles<br> "
"**: If you supply a name, the query will be saved off
and available as a link "
"in the sidebar"

and in html, the string is :
   *: The "all text" field will look in message bodies
and issue titles<br>
   <span
tal:condition="python:request.user.hasPermission('Edit',
'query')">
   **: If you supply a name, the query will be saved
off and available as a link in the sidebar

I tried to put this string in msgid, but that didn't
seem to work, may be because of the tal condition that
cannot be in a .po file. So, I found a way by by using
two spans in issue.search.html :

   <span i18n:translate="">
   *: The "all text" field will look in message bodies
and issue titles<br>
   </span>
   <span
tal:condition="python:request.user.hasPermission('Edit',
'query')" i18n:translate="">
   **: If you supply a name, the query will be saved
off and available as a
link in the sidebar
   </span>

I will attach a patch correcting that bug in
issue.search.html and in each locale.