Tales of the Bloodstone Lands
Given enough free time and the services of an instructor, a character can learn a language or pick up proficiency with a secondary skill (tool proficiency).
A character may also learn the use of a specific weapon for which they don’t have proficiency, such as an exotic weapon, or a character limited to simple weapons learning to wield a rapier, for example.
Receiving training in a language or tool typically takes at least ten workweeks (100 days), but this time is reduced by a number of workweeks (10 days) equal to the character’s Intelligence modifier (an Intelligence penalty doesn’t increase the time needed). Training costs 25 gp per workweek.
In addition, any day spent with a trainer counts as two days for the purpose of acquiring a secondary skill or language. If a character is providing the training, they must use their “downtime day” training the student.
At the end of the time spent training, the character has attained mastery of the secondary skill (tool proficiency), proficiency in a new weapon, or basic fluency in a new language.