I use 'jf' instead of 'jk', so I can type "Dijkstra" occasionally, not to mention they're both on the home row, under your index fingers, typically have physical nubs on the keys, and no words in any language I can find contain the letters 'jf'.
As for jf: I searched through the pdf of a 1300 page Norwegian dictionary, and the single word in there containing jf is the abbreviation "jfr." which is the same as the abbreviation "cf." (confer) in English. So jf appears to be a very good choice.
Also jfr isn't a particularly rare abbreviations - so ymmv. At any rate, with how words can be combined in Norwegian, there's generally no good safe choice...
Another example, in line with the IKEA theme: "dusjforheng" ("shower curtain").
for abb in jk jf;
do
echo "Occurances of ${abb}:"
grep ${abb} -c \
/usr/share/dict/{bokmål,nynorsk,british*,svenska}
done
Occurances of jk:
/usr/share/dict/bokmål:90
/usr/share/dict/nynorsk:39
/usr/share/dict/british-english-insane:20
/usr/share/dict/svenska:70
Occurances of jf:
/usr/share/dict/bokmål:31
/usr/share/dict/nynorsk:32
/usr/share/dict/british-english-insane:0
/usr/share/dict/svenska:0