You can't measure importance of a language within the science of linguistics.
Since it is clearly subjective you have to use non-linguistic and/or non-scientific methods like drawing up a good old fashioned list of pros and cons (criteria). You can choose (again subjectively) whether to treat each criterion as equal, or to assign them weights. You can choose whether to just allow yes/no answers or a range with positive and negative extremes and a neutral zone in the middle.
Here is an example as requested for a language somebody might find important. I've weighted each criteria equally and allowed a range between -20 and +20 for scores.
(?
means "I have no idea" so is equal to 0
)
Chinese Arabic Swedish Georgian Avar
Could I make more money if I learn it?
+10 +5 +5 -5 -10
Can I quickly learn its written form?
-10 -10 +10 -5 -5
Is it spoken by hot chicks I would like to chat up?
+5 +5 +10 +10 ?
Might its utter differences to languages I already know offer tangible linguistic insights?
+5 +10 +2 +10 +20
Can I practice it with the people in my local convenience store or ethnic restaurant?
+10 0 0 0 0
Could I have a fun and safe holiday visiting a country where it's spoken?
+5 +5 +7 +10 -20
Will it impress the hell out of people I randomly bump into who speak it?
+10 +10 +10 +10 +10
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+35 +25 +39 +30 -5
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Men svenska är tråkigt. Kan jag välja något annat? (-: