Getting a funding position as a foreign MSc student in Finland
My experience is from Jyväskylä, not Helsinki, and in pure mathematics. Things might work differently, but maybe this is still useful.

Teaching assistants

These were typically recruited among the better students. Much of teaching happened in Finnish, especially at the lower level courses, and as such, knowledge of Finnish was assumed.

Getting such a position right at the beginning might be difficult, but if you are skilled, and have good command of English (or even better, know or start learning Finnish), it might be possible. I doubt Swedish would help quite as much.

Research assistants

Research assistants were recruited among promising students and often involved writing their master's thesis, but might also involve research. Often these were summer jobs.

Connections

If you, or someone at your university, has connections to the faculty, you should try to ask via them. In particular, if someone can recommend you as an excellent student and a potential PhD student, the chances that someone will arrange a position for you will vastly increase.

Ask

It never hurts to ask. Contact information: https://www.helsinki.fi/en/programmes/master/data-science/contact

Scholarships

If you have not looked at it, the scholarship webpages of the university might be helpful: https://www.helsinki.fi/en/admissions/scholarship-programme