Sphinx's i18n feature needs "node.source" and "node.line" value for translation process.
(see also: https://sourceforge.net/p/docutils/patches/100/)
Sphinx extract translation target message from parsed doctree. Sphinx uses nodes that are instance of nodes.TextElement and have source information as translation target messages:
Docutils parses admonition directive as nodes.admonition. nodes.admonition has child node as nodes.title but it doesn't have source/line information.
Could you set source information to term node? I attach a patch to do it.