ob-napkin
supports Python syntax for sequence diagrams along with the plain plantuml code. Note that it uses Plantuml server, which means that it does not require to install JAR file and usually faster than invoking JVM.
plantuml.jar
from official download page and save it at /home/you/path/to/plantuml.jar
2.) install plantuml-mode
via melpa.
3.) in *scratch*
execute
(setq org-plantuml-jar-path (expand-file-name "/home/you/path/to/plantuml.jar"))
(add-to-list 'org-src-lang-modes '("plantuml" . plantuml))
(org-babel-do-load-languages 'org-babel-load-languages '((plantuml . t)))
4.) put some uml in your org file e.g.
#+begin_src plantuml :file my-diagram.png
title Authentication Sequence
Alice->Bob: Authentication Request
note right of Bob: Bob thinks about it
Bob->Alice: Authentication Response
#+end_src
5.) export e.g. with C-c C-e h o
#+BEGIN_UML
Alice -> Bob: Authentication Request
Bob --> Alice: Authentication Response
#+END_UML
While the org file is exported to HTML or other formats, PlantUML is called to generate image and replace the block in place.
Thanks to Ian Yang for the work done. This method, however, is considered obsolete:;; OBSOLETED, use ob-plantuml.el bundled in org instead.( A citation from https://www.emacswiki.org/emacs/org-export-blocks-format-plantuml.el )