Build a sentiment model from scratch
What you're describing is indeed the traditional approach for building a sentiment analysis system, so I'd say it looks like a reasonable approach to me.
I'm not up to date with the sentiment analysis task at all, but I think it would be worth studying the state of the art for several reasons:

There might be more recent, better approaches
There might be datasets in the languages you're interested in, and if there is that could save you a lot of time. Check if there are any shared tasks about this, they often provide annotated datasets.