G/L Entries are ALWAYS in LCY - however, you can enter transactions in G/L Journal in other currencies, too - but they’ll be recalculated to LCY immediately.
G.L Entry table has no fields for Amt in other currency than LCY, so after posting a G/L Journal, you loose trace of fact, that transaction was not in LCY. Such functionality exists in, e.g. Bank Account Ledger, so those can be balanced both in LCY and FCY, but even then, when these are posted to GL, currency info is lost in GL, it remains only in BAL.