This idea would be perfect for a youth group preparing for a missions trip to a foreign, non-English-speaking country.
The objective of this is to get the students planning to go on the trip to learn as much of the target language as possible. Have each student collect pledges per each word of the language they can learn in a specific time frame (one month might be a good time frame). Then, encourage them to study hard for that time frame. There are many free online language dictionaries that could be used as resources. (Make sure they’re focusing on learning appropriate language, not swearing and bathroom humor.) At the end of the time, hold a service where the youth can show off their newly attained language skills and get totals on how much they need to collect in pledges. This is totally free, just requires the time commitment from the students, and gives them skills they’ll need for their trip while raising money at the same time.
Here’s a hint: play this up as a competition (who can learn the most words, who can say the most complete sentences, who learns to say the most from a list of key phrases such as “Jesus loves you”, etc.). Youth love a good competition, and this will convince them to work even harder for it.