Rosetta Stone software is quite expensive, but the general approach is to create a seemingly immersive environment by not using translation. At the basic level, it seems they use flashcards to learn vocabulary and basic grammar, but rather than translation-based flashcards, they are flashcards that associate images with words in the target language.
There’s more to Rosetta Stone than this, certainly. It’s an approach that has some obvious merit for beginners, alongside some pretty severe limitations for emergent speakers. One criticism of the company is that they tend to recycle images between languages, so that they are not as culturally relevant.
Some elements of this approach can be done using spaced repetition software such as Anki. I’m working on a library of culturally-relevant concrete nouns in Czech with matching images. This is pretty laborious to assemble, since I’m looking up words in an English-Czech dictionary, cross-referencing them in the Czech-English dictionary, looking up words in English and Czech Wikipedia to get a sense of their taxonomy, and doing a Czech Google Image search to see what the most common Czech-language results are for it. This list is very loosly based on this one for no particularly good reason, but modified for appropriateness and cultural relevance. For the time being, this focuses on concrete nouns, although visually illustrable verbs might be a good addition. I suspect that I’ll start a translation-based deck for abstract nouns and concepts (such as grammatical terms) or common phrases, but I want to keep the main Anki deck in Czech only. So later, with greater comptence, I might add text-based flashcards that, for example, give a Czech definition of a Czech term.
Having learned French badly, I’m determined to learn the morphological categories of words alongside the word itself as much as possible. Since the Czech word jeden/jedna/jedno “one” is inflected for gender, all of these cards will use phrases like jedna kočka to have gender indicated is a more naturalistic phrase. This won’t always be possible, since some Czech nouns are inherently plural. Also, good Czech dictionaries also list the genitive form to make clear the declension paradigm, and I will do this if possible. Noun gender, noun declension and verb conjugation are probably going to be the most difficult grammatical paradigms for me, but I’ll worry about the latter two in a bit.
After building this image library, I’m going to use PocketMod to create some simple children’s picture books in the target language. These needn’t be more difficult than eight pages of Co je to? To je kočka “What is this? It’s a cat”. Our families do a lot of reading together, and reading is a good way to interact, build vocabulary, and increase comprehensible input. Children past the age of two can acquire hundreds or thousands of words per year, depending on how much they read, but we have to start smaller.
In the meantime, look at the Little Czech Primer.