Translation & docs update

gui/assets/lang/lang-bg.json

gui/assets/lang/lang-ca.json

gui/assets/lang/lang-ca@valencia.json

gui/assets/lang/lang-cs.json

gui/assets/lang/lang-de.json

gui/assets/lang/lang-el.json

gui/assets/lang/lang-en-GB.json

gui/assets/lang/lang-es-ES.json

gui/assets/lang/lang-es.json

gui/assets/lang/lang-fi.json

gui/assets/lang/lang-fr.json

gui/assets/lang/lang-hu.json

gui/assets/lang/lang-it.json

gui/assets/lang/lang-ja.json

gui/assets/lang/lang-ko-KR.json

gui/assets/lang/lang-lt.json

gui/assets/lang/lang-nb.json

gui/assets/lang/lang-nl.json

gui/assets/lang/lang-nn.json

gui/assets/lang/lang-pl.json