It’s true that 70% of a language is about ~100-300 words. In linguistics this is called the “core sight set”. If you’re in a pinch traveling I recommend asking an AI for the 300 most frequent word core sight set and cramming these with Anki. You can get gist with about 10 hours of study and be much more useful than 100 hours of Duolingo. With the core sight set and a generous amount of loan words and gesticulation you can communicate practically any necessity to anyone. It will by no means be elegant or poetic but it gets the job done reliably. It’s the 10,000 word long tail of vocabulary where a language shines but it’s the first 300 where it lives and breathes.

I tried this is it didn't work. The most common words are the most versatile too and need context

You can learn word "investigation" without context, but not get or set

The focus on a small set of core vocabulary is one of the main principles of the Pimsleur method, along with a strict spaced repetition format. When I travel to a new country I always spend about 15-20 hours beforehand doing the 30-minute Pimsleur lessons, just to pick up basic survival vocabulary. I've always been satisfied with the results.

I cannot find anything on google nor on google scholar under "core sight set" that has anything to do with language.

In fact the term does not appear to exist at all.

Frequency lists are very useful but learners need context in order to use them, because little word atoms like prepositions, pronouns etc are heavily over represented in the core set. So make sure to study how to use those, and master the core to be/to have verbs too. Some languages have two verbs that roughly translate as to be so you need to crack that too.

Beware free lists on Ankiweb. They are very variable in quality. Frankly better to build your own.