templates: Use 'en' locale

templates: Use 'en' locale

As discussed at [1], the UI was originally written in Australian English
but as it's been through a couple of pairs of hands since the chances
are things are more than a little messed up. Just use 'en' as our locale
rather than 'en-US', 'en-AU' or anything else.

[1] https://lists.ozlabs.org/pipermail/patchwork/2019-November/006342.html

Signed-off-by: Stephen Finucane <stephen@that.guru>
Suggested-by: Daniel Axtens <dja@axtens.net>