I think adapting services like https://saysomethingin.com, can help and I think if you substitute every Welsh word for the corresponding word in the language you're trying to teach, it could work well for a number of languages.

I have learned several languages, very similar ones, and very different ones to my mother tongue. There are of course differences, but as you are asking for a general strategy, here it goes:

The distinction between basic grammar and a lot of vocabulary is important. A native speaker will understand you, regardless if you use advanced grammar or not. However, if you want to talk about daily stuff, knowing the words for each of these things is important. Example:

I like eat banana is gramatically incorrect, but you get your point across.

or

I eat long-shaped-thing-that-I-am-trying-to-show-you-with-my-hands will make it sound like you like to eat any possible phallic object.

My scheme: Learn basic grammar Learn most used nouns, verbs Listen to podcasts (e.g. A.J. Hoge's podcasts) The better you want to know language the harder should be every stage