In the first place, the whole ceremony could be conducted in a different language (including Gaelic or old Scots) as long as the couple, the Celebrant and the witnesses understand what is being said, using the services of an official translator if necessary. More importantly, the legal declarations do not have to be made in English, as long as they are clearly understood by the couple, the Celebrant and the witnesses.
Most of the ceremonies I conduct are entirely in English, with the odd bit of Scots thrown in, but I do get asked to do bi-lingual ceremonies as well, and they are great fun.
I'm lucky that I've lived in France, so I'm reasonably fluent, and my work as a producer and director took me round a lot of the world, so I've learned how to ask for two beers in most languages apart from Arabic - it's generally not a good idea to ask for a beer in Saudi Arabia... I know that my accent in most languages is considered to be good, and I really enjoy speaking knowing that all the guests are being included in the ceremony.
To date, I've incorporated French, Spanish, Catalan, Basque, Italian, Dutch, Polish, German, Japanese and Russian elements in weddings I've conducted. Je parle francais, ed io parlo italiano, anche espanol, pero no hablo mucho. Ich sprach kein Deutsch, Ich bedaure, but I have a smattering of Polish, at least enough to say przepraszam, czy mowisz po Angielsku?
What you should consider is just which parts of the ceremony need to be in more than one language.
If it all needs to be translated, then it's a good idea to keep the ceremony short in the first place, but my usual advice is that we include a few elements that I speak in both languages, but bear in mind that it tends to take longer to say the same thing in both French and German.
Usually I will speak in both languages, but I'm entirely happy to have a native speaker along side me, translating paragraph by paragraph.
Another way to do it is to have someone out in amongst the audience simultaneously translating for the benefit of those guests who have no English at all - if they all sit together, that can work well.
Alternatively, if there are a lot of guests who have no English at all, then it would be worth giving them sealed copies of the ceremony that they can read at the same time as I speak, but I would generally counsel against that is it means that the guests will sit there reading, rather than watching and listening.
So - I hope that's a help. Au revoir, auf wiedersehen, da svedanya, ci vediamo, (yes, yes, shut up, we get the point!)