I know this is a little broken or over powered, but please consider the following from a RAW or RAI point of view. Please make sure your answer reflects which you mean.
If a character happens to learn a sign language (currently the only one listed is drow sign language and is a secret language) would they be able to use that for their spell casting in place of verbal spell components?
Under casting a spell
To cast a spell, you must be able to speak (if the spell has a verbal component), gesture (if it has a somatic component), and manipulate the material components or focus (if any). Additionally, you must concentrate to cast a spell.
And since there is a silent language which is spoken by hands according to language rules
You are skilled at working with language, in both its spoken and written forms.
If there was some silent language, like a stone language (not terran) would that also work? As long as you "speak" can you fulfill that component?