Officially a translation for the current default syntax is support to
be plain text with just MessageFormat syntax support for (again plain
text) parameters. That's the behavior you will alway get with the
{{translation}} macro for example. All the translations you see with
html (or worst wiki syntax) are very bad hacks.
The only hack (there is no clean one) I can see (which does not work
in all cases) is to use the {{translation}} macro in your second
translation.
MailArchiveCode.LoadingSessionClass_maxMailsNb=Max number of mails to load
mailarchive.loadingSession.fields.maxMailsNb.label={{translation
key="MailArchiveCode.LoadingSessionClass_maxMailsNb"}}.
We really need support for variables in localization modules
(something like mailarchive.loadingSession.
fields.maxMailsNb.label={{MailArchiveCode.LoadingSessionClass_maxMailsNb}})
) and I designed it in order to make easy to support it later but the
localization module went lower in my priority list these days.
In the meantime something you could do on your side is override the
component
https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-core/xwi…
which is the current default and only syntax and put there your own
placeholder system.