This thread may be useful.

This website allows you to search for particular strings in words. A real gem, thanks to Craig Eliason, I'm going to keep referring to it. One example given: the Finnish word for 'biosphere' is 'biosfääri' with two adieresis-es.

I've worked with a printer called Fälth & Hässler. Just lc that and you're on your way!

A few German words:

fähig [able, capable]
fällig [due]
fällen, gefällt [to fell, felled]
fälschen, gefälscht [to falsify, falsified]
färben, gefärbt [to color, colored]

There are a lot more. The most common one might be ‘gefällt’ – as in ‘Gefällt mir’ (‘Like’ on Facebook).

Charles, you might also find the German Affäre of interest — ff-lig (which, if I’m not mistaken, is actually permissable in this case) + ä.

In Swedish, biosfär (biosphere) might suit your need.

Thanks guys.

thanks to Craig Eliason

To give credit where it's due, James Puckett was the first to link to it from Typophile.

A veritable treasure-trove of f-ing umlauts!;-)
Thanks to everyone!