Какой предлог выбрать?

[VictorB]

Контекст:
Немец и поляк (или китаец и узбек - совершенно неважно), сидят в тюрьме в одной камере (или оказались на необитаемом острове). При этом, каждый может худо-бедно изъясняться на языке своего сокамерника (соостровитянина). И вот, чтобы зря время не терять, договариваются они среди прочего продвигать свои знания в иностранных для себя языках путем поочередного использования их в интеллектуальных беседах друг с другом. То есть они договорились по очереди практиковаться друг на друге в иностранном для каждого языке. Или друг с другом? Видно, что у меня и на родном языке тут затруднение с выбором предлога.
Так какой предлог выбрать при изложении этой идеи на английском, with или on:

They agreed to took turns to practice their second languages with each other.

OR

They agreed to took turns to practice their second languages on each other.

?

[VictorB]

Так какой предлог выбрать при изложении этой идеи на английском, with или on

Sorry, I should have searched for the collocation with the verb practice in British spelling, i.e. "practise". Now that I've done so, the question's been solved: There's no alternative to "on".
Anyway, I'll leave the thread active for anyone who may have similar questions of their own. As for me, nearly half of what I have to check and re-check is about prepositions in collocations, so I'm going to use it myself in future more often than not:-)

[Brow]

Practice language with ...

[VictorB]

Practice language with ...

Why not, if it is to be used in the context of practicing a foreign language together with a partner/partners to whom it's foreign too?
My context is somewhat different, no?

[Brow]

practicing a foreign language together with a partner

Use with
But you might as well practice English on Skype , Facebook or ... on a daily basis )

[VictorB]

But you might as well practice English on Skype , Facebook or ... on a daily basis

What about this:
I learned haidressing by practicing on my sister?

[VictorB]

But you might as well practice English on Skype

Re: on Skype
By this link I only mean that there's always more than one way to skin a cat:-)