A great way to incorporate foreign language into your HH is through Inter-Wizard Relations. Are your students studying French? Why not have all your french lessons come from Beauxbatons?
Dumbledore can send the initial Owl, explaining that the Ministry would like to improve relations between the two schools, so all Hogwarts students (and Homeschool Charters) shall begin by studying their language - and they ours.
From then on have your lessons come from some Professeur of something or other at Beauxbaton. You can have this fall under Advanced Charms, or simply Lanugages - that's up to you.
If you're just starting out, keep it simple. Start with vocabulary words, and simple sentences that pertain to your everyday life. Things that you and the kids will be saying regularly: bed, door, car, etc - and - put away your toys, i love you, etc.This is a wizarding school, so don't leave out wand, owl, etc.
For families who've been studying a language for some time, it's as simple as having lessons and Owls coming from the other Wizarding school. Though, as, with those new to the study, add wizarding phrases: Has anyone seen my cauldron?, or the ever-useful You probably should not eat that, as I made it in Potions class.
Don't forget to make it fun! Kids just starting out can label everything with little squares of parchment. Seeing the words on the items is a great way to reinforce what was learned. If you're going to write out sentences (in which they'll insert the correct vocabulary word) make it Hogwartsy: Moaning Myrtle lives in the u-bend of the ____. That's vastly more fun than In the bathroom we sit on the ______.
More advanced students need to have fun too! Have them write letters in the other language. Perhaps a letter to Charlie Weasley for advice on rearing dragons, or to Professor Flitwick asking for the charm that enchants spinach into tasting like chocolate? (assuming you know more of the language than your students, don't forget to write back!)
Older kids can write to penpals; either those who are also learning the language, or those from the country in question. Make sure to find penpals from reputable places (to ensure safety), and stick with other HP fans.
Inter-Wizard Relations can carry over into other areas of HH. Teaching Kitchen Potions? They can plan and cook a meal from the other country. Herbology? Plant a French/Spanish/Etc herb garden. As usual, the only limits are your imagination.
We here at HH are just beginning our Inter-Wizard Relations. Our students shall be receiving lessons from a Wizarding school in Spain. Due to still being behind, we were forced to explain what was going on - and of course blamed Errol for Dumbledore's Owl having not yet arrived. Errol does come in handy ;-)
I hope we've given you a few ideas to get you started. Oh! Don't forget to use parchment instead of boring Muggle paper.