[TASK] Streamline buttons in Language module

Apply the btn classes to the action buttons in the Language module to
make the look and feel more consistent.

Resolves: #66509
Releases: master
Change-Id: I5c4b2b2318d1ae7fac20f4b4021e4911044752c4
Reviewed-on: http://review.typo3.org/38842


Reviewed-by: Felix Kopp <felix-source@phorax.com>
Tested-by: Felix Kopp <felix-source@phorax.com>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>