Fixed by having renderer.action_button return a formated string, using the translated text.

Fixed in HTML/bs3 and ui5 renderer

in request GET /api/main_html
TRACEBACK:
  File "/home/tonis/Projects/lino/2.7/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/tonis/Projects/lino/2.7/local/lib/python2.7/site-packages/django/views/generic/base.py", line 68, in view
    return self.dispatch(request, *args, **kwargs)
  File "/home/tonis/Projects/lino/2.7/local/lib/python2.7/site-packages/django/views/generic/base.py", line 88, in dispatch
    return handler(request, *args, **kwargs)
  File "/home/tonis/Projects/lino/src/lino/lino/modlib/openui5/views.py", line 510, in get
    request, extjs=settings.SITE.plugins.openui5)
  File "/home/tonis/Projects/lino/src/lino/lino/core/site.py", line 3401, in get_main_html
    request, 'admin_main.html', **context)
  File "/home/tonis/Projects/lino/src/lino/lino/modlib/jinja/__init__.py", line 84, in render_from_request
    return template.render(**context)
  File "/home/tonis/Projects/lino/2.7/local/lib/python2.7/site-packages/jinja2/environment.py", line 1008, in render
    return self.environment.handle_exception(exc_info, True)
  File "/home/tonis/Projects/lino/2.7/local/lib/python2.7/site-packages/jinja2/environment.py", line 780, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/home/tonis/Projects/lino/src/lino/lino/config/admin_main.html", line 1, in top-level template code
    {% extends "admin_main_base.html" %}
  File "/home/tonis/Projects/lino/src/lino/lino/config/admin_main_base.html", line 101, in top-level template code
    {% block admin_main_items %}
  File "/home/tonis/Projects/lino/src/lino/lino/config/admin_main_base.html", line 103, in block "admin_main_items"
    {{ i.render(ar) }}
  File "/home/tonis/Projects/lino/src/lino/lino/core/dashboard.py", line 106, in render
    return self.render_request(ar, sar)
  File "/home/tonis/Projects/lino/src/lino/lino/core/dashboard.py", line 69, in render_request
    s += tostring(ar.show(sar))
  File "/home/tonis/Projects/lino/src/lino/lino/core/requests.py", line 742, in show
    return doit()
  File "/home/tonis/Projects/lino/src/lino/lino/core/requests.py", line 737, in doit
    show_links=show_links)
  File "/home/tonis/Projects/lino/src/lino/lino/core/renderer.py", line 151, in show_table
    return tostring(self.table2story(*args, **kwargs))
  File "/home/tonis/Projects/lino/src/lino/lino/core/renderer.py", line 175, in table2story
    return ar.table2xhtml(**kw)
  File "/home/tonis/Projects/lino/src/lino/lino/core/tablerequest.py", line 401, in table2xhtml
    self.dump2html(t, self.sliced_data_iterator, **kw)
  File "/home/tonis/Projects/lino/src/lino/lino/core/tablerequest.py", line 440, in dump2html
    recno, columns, obj, sums, **self.renderer.cellattrs)
  File "/home/tonis/Projects/lino/src/lino/lino/core/tablerequest.py", line 566, in row2html
    v = col.field._lino_atomizer.full_value_from_object(row, self)
  File "/home/tonis/Projects/lino/src/lino/lino/core/store.py", line 378, in full_value_from_object
    return self.vf.value_from_object(obj, ar)
  File "/home/tonis/Projects/lino/src/lino/lino/core/fields.py", line 525, in value_from_object
    return m(obj, ar)
  File "/home/tonis/Projects/lino/src/lino/lino/core/model.py", line 854, in workflow_buttons
    return self.get_workflow_buttons(ar)
  File "/home/tonis/Projects/lino/src/lino/lino/core/model.py", line 897, in get_workflow_buttons
    return E.span(*l)
  File "src/lxml/builder.py", line 222, in lxml.builder.ElementMaker.__call__ (src/lxml/builder.c:3413)