How do I convert an array to string using the jinja template engine?

You can use regular python in jinja tags. an obvious choice for some simple cases is str.join:

>>> jinja2.Template(r'{{ " ".join(bar) }}').render(bar='baz')
u'b a z'.

You can also iterate over sequences in jinja with a for block:

>>> jinja2.Template(r'{% for quux in bar %}{{ quux }} {% endfor %}').render(bar='baz')
u'b a z '

Actually you are almost there, for join with space, just put it like this:

{{ tags|join(' ') }}

see the jinja docs for more details