Languages can be practically impossible to learn without knowing another specific language first when all teaching materials, dictionaries, and grammar references are written exclusively in that other language. This is not about linguistic similarity, but about documentation and access. For example, many minority or endangered languages like Ainu, Sami languages, or Faroese have their most complete resources in Japanese, Norwegian/Swedish, or Danish respectively, making them effectively locked for learners without those keys.
There are a few distinct types of language-locked languages. The first is resource-locked: the only decent grammars and dictionaries are written in a dominant national language. Faroese is a classic case, while theoretically learnable from English, the vast majority of teaching materials and reference works run through Danish. Approaching it any other way leaves you with almost nothing. Similarly, many indigenous languages of Siberia have their best documentation in Russian, and many in Latin America depend on Spanish.
The second type is community-locked: the language is spoken by a small group, and the only way to learn it is through immersion within that community, often requiring a local lingua franca. For instance, learning a language like Ainu effectively requires Japanese because the remaining speakers and cultural context are in Japan.
The third type is script-locked: languages written in a script that has no standard transliteration, and all learning materials assume you already read that script. Classical Tibetan, for example, is often taught through Chinese or Sanskrit, and many resources assume familiarity with those.
If you want to learn such a language, first research what the “key” language is. Check Wikipedia’s list of endangered languages and look at the bibliography of any grammar book. Search for “learn [language] in [key language]” to see if materials exist. If you don’t know that key language, you may need to learn it first, at least to a reading level. Alternatively, look for newer resources in English or other major languages, as some communities are working to create more accessible materials. Be honest about the effort: learning a language-locked language often means learning two languages, or accepting a very slow, patchwork path. The reward is access to a unique linguistic world, but the tradeoff is significant time and frustration.