Dutch

Both, in different ways. Dutch shares West Germanic ancestry with both. Grammatically, Dutch sits between them — V2 word order like German but a much simpler case system that has reduced essentially to vestigial pronoun forms (closer to English than to German). Vocabulary overlaps heavily with both. A Dutch speaker can read German with effort and English with relative ease, and the same is true in reverse for German speakers reading Dutch.

Flemish is the everyday name for the Dutch spoken in Flanders, the northern Dutch-speaking region of Belgium. It's not a separate language from Standard Dutch — they share a written standard — but the spoken Flemish varieties have distinctive vocabulary, prosody, and pronunciation. Flemish speakers and Netherlands Dutch speakers understand each other readily, though the cultural and accent differences are noticeable.

Two: common (de) and neuter (het). The historical masculine and feminine genders merged into a single 'common' category centuries ago. Articles, demonstratives, and adjectives mark the common/neuter distinction. The system is much simpler than German's three-gender setup but still requires learning the gender of each noun, and there are no fully reliable rules to predict which gender a noun belongs to.

Yes — Afrikaans descended from 17th- and 18th-century Dutch settler speech in southern Africa and developed into a separate language. Dutch and Afrikaans share most of their vocabulary and grammar, with Dutch speakers able to read Afrikaans easily and follow conversation with some adjustment. Afrikaans has simpler grammar (most case marking and verb conjugation has been levelled) and distinct spelling conventions.

One of the more accessible major languages for English speakers. The grammar is closer to English than German is — minimal case system, simpler verb conjugation. The vocabulary overlap with English (and German) is heavy. The hard parts are the V2 word order, the gender system, and pronunciation: the velar fricative, the diphthongs, and the Dutch r have all been known to challenge learners.