@shani If you want want help with textbook selection (or any other advice) don't hesitate to ask! Of course, if they're Japanese-German textbooks that don't have an English counterpart then I won't be of much help, but if not I'd be happy to!

From my experience, picking the right textbook is pretty crucial, as some are incredibly dry and lifeless, which is a massive turn-off. I found Japanese for Busy People sat firmly in the lifeless category, while the Genki series felt really engaging. There's a subtle charm and humour to them that makes the reading more memorable and compelling, and I feel the serious 'linguistic' side of things strikes the right balance between readability and rigour (for a beginner's textbook, anyway).

If you want to go deeper into things, I think Tae Kim's Guide to Japanese is a useful online resource to be aware of, which can give you a different angle on various grammar points.

And as for general approach, it's widely regarded that the sooner you can learn the Kana (Hiragana and Katakana), the better. It actually caught me off guard how achievable that actually was, as the different script is one of the most initially intimidating things about the language for all readers (after all, how are you meant to gauge how hard something if you've got no idea what it says). Personally, I don't think 100% romaji textbooks should even exist (unless they're really short), as it's just postponing such a necessary - and actually not all that difficult - part of the language.

Most games come with Hiragana written above the Kanji (known as 'Furigana') which means that with a solid grip on Hiragana and Katakana, you should be able to at least pronounce everything in the game, even if you can't understand it. Crucially, it'll make looking things up easier (although that that might not be such if issue if you're using the Translate App).

As for Kanji, people have different approaches, and even these approaches change over the course of a person's studies, but most textbooks should have a decent drip-deep of characters that are both simple and common. The Kyouiku Kanji list can provide some people with a basic ordering of characters to learn first, but needn't be stuck to too religiously. Some people like to do 'Kanji Study' as if it's almost it's own separate subject, but I find the further I isolate it from the rest of my language knowledge the harder things are to remember. After all, if you know a character but not any words that it features is, then you can't use it as the building block of a language, which is exactly what a character is supposed to be.

Anyway, the two Kana scripts should keep you busy enough before you have to worry about any of that.

I hope you find a textbook you get along with!