Loremaster 5 was always granted access to a secret language even before the recent changes. Except, you could also pick a less-secret language.
Now that LM 5 ONLY gets access to secret languages, there is an increased incentive for Loremasters to pick languages that should be lesser known because they simply don't get to choose something less rare.
I'd suggest just allowing LM 5 select any language, even ones that aren't rare or secret. This allows more flexibility and alleviates the risk of secret languages continuing to be common just a little more.