Just some two cents: A good rule of thumb in Finnish is that the negation is almost always stronger and takes precedence even if there are affirmative words (kyllä, joo) mixed in.
– Haluaisitko tilata lehden? (would you like to order a magazine?)
– Juu en tilaa (oh, no I won't)
– Mennäänkö pilkille tänään? (are we going ice fishing today?)
– No ei kyllä mennä! (no we really aren't going!)
A lot of people spell the affirmative word 'yeah'. A Google search reveals 780,000,000 results for 'yeah', while 'yah' gets only 150,000,000, much of which refering to things like the Hebrew Yahweh, the Egyptian Yah/Lah, an interjection expressing disgust, etc.
Personally, I've seen 'yes', 'yeah', 'yep', 'yup' and old-fashioned 'yea', but never 'yah'.