Maximize your Intelligence modifier
Outside of time travel antics1, the only2 way to decrease this time by the rules is to increase your Intelligence modifier, since the rule just says:
this time is reduced by a number of workweeks equal to the character’s Intelligence modifier (an Intelligence penalty doesn’t increase the time needed)
There are two cap levels for ability scores (p. 173, PHB):
A score of 10 or 11 is the normal human average, but adventurers and many monsters are a cut above average in most abilities. A score of 18 is the highest that a person usually reaches. Adventurers can have scores as high as 20, and monsters and divine beings can have scores as high as 30.
So how can you increase you Intelligence ability modifier?
- First, simply selecting high Intelligence during character creation, and by Ability Score Increases or Intelligence boosting feats. This will take you to Intelligence 20, for +5. (Headband of Intellect is not so useful here, as it just sets it to a fixed 19; the Ioun stone of Intellect, can stand in for one of the ASIs).
Then, there are things that can boost your ability score beyond 20. Since you are normally capped at 20, the thing needs to tell you it can boost the score above 20. And since you need it for the entire time, only those with an ongoing effect over long time periods are useful here
True Polymorph on yourself, held until its permanent, for example into a Blue Abishai or Pit Fiend can get you up to 22 Int without any item. The highest intelligence monsters like a Solar with Int 25 or individual monsters like Fraaz Urb'luu with 26 Int are unfortunately off-limits for the spell, either because they are individuals, or because their CR is above your maximum attainable level of 20.
The Star card of the Deck of Many Things: "Increase one of your ability scores by 2. The score can exceed 20 but can't exceed 24."
A tome of clear thought is probably the best of all these, as it can increase it by 2 along with your maximum (presumedly, up to the upper cap of 30). All you need is 5 of these very rare items to get it to 30. (Or you can try this trick, with a single one).
So, theoretically, you can boost your Intelligence modifier to +10, at which point it would take you "at least ten workweeks" less ten weeks to learn something. Purely mathematically, you could get to 0 weeks, and learn all languages and tools immediately. In practice, the "at least" gives your DM ample leeway to rule you need at least one week of study to learn a tool or language.
An entirely different approach would be to not learn anything the hard way, but sidestep the entire issue by picking a race such as a Shadar-Kai elf (p. 31, Mordenkainen presents: Monsters of the Multiverse), which allows you pick up a skill of choice for the day:
Whenever you finish this trance, you can gain two
proficiencies that you don't have, each one with a
weapon or a tool of your choice selected from the
Player's Handbook. You mystically acquire these
proficiencies by drawing them from shared elven
memory, and you retain them until you finish your
next long rest.
1 OK, OK, so you want time travel antics. Find a friendly sphinx that sends you back in time 10 years whenever you like. Now, all you need is a way to stop yourself from aging, and you have all the time in the world to learn whatever tools or languages you like.