「然る」の正しい発音は「しかる」ですか、「さる」ですか？

See a translation

Both are right, and mean "this" or "correct".
Since these words actually come from classical Japanese (and moreover, mostly used in translation from classical Chinese into Japanese in ancient eras), they are hardly used in modern Japanese except a few poetic expressions.
然（しか）るべき shikaru-beki → something should be or should be down. In some classical Japanese text this also pronounciate as "sarubeki", which is the adjective formation of "sarubeshi".
然（さ）もなくば sa-mo-naku-ba (yes, this is actually a phrase of 4 words!) → if not, otherwise.
e.g. 然もなくば然るべき処置を取る。 Sa-mo-naku-ba shikaru-beki shochi wo toru.
Otherwise (we) will take a proper action.

I hope this can help, though this is really a difficult question and I suggest you to ask your teacher.
By the way I am planning to learn Czech :-)