main.cpp: fix broken build for libgit2 0.21

799e22ea0c3f20 in libgit2 renamed a function, which
breaks our build. To fix that we check the current version
and if the USE_LIBGIT21_API definition was used.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Latest string and translation updates

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>