Make Reporting admin section translatable (#1549)

* Make Reporting admin section translatable

And translate it into english and french

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

* Make subject of emails translatable and improve french translation

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

* Make error pages translatable and translate them in english and french

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

* Translate site setting section

* Insert instance in registration emails and improve them a bit

Signed-off-by: Thomas Citharel <tcit@tcit.fr>