Learning a language very similar to one you already know

[Gwenhwyfar]

So it would be useful for me to be able to at least understand a certain language. I already speak a very similar one and a somewhat similar one. That means going to an evening class and starting from scratch would be a huge waste of time, because I already understand quite a bit when people speak slowly or write.
On the other hand, my similar language is a bit rusty now so getting a book aimed at natives of that language would probably be too hard.

Any suggestions? Bearing in mind, I don't really need to be able to speak and write the new language and know all the grammar rules, just understand (a bit more) when people are speaking.

Are there any mirror books for learners available? Even if it would be books for natives of that language learning English.

Plus, anything where you can get video in the target language with subtitles in the same language, so YouTube can be a good start

Find one of those very formulaic reality TV competition or dating shows in the target language and watch it with the subtitles in the same language. One of those where they tease what will happen, then it happens, then they recap, then people talk about their feelings about the minor event. Have the radio on in the background when you can (not at the same time).
In a month or two, start an intermediate or advanced class. If you are missing any grammar concepts, you’ll find it easy enough to teach yourself.
(I’ve done this with three Romance languages to fluency, it works for me.)

Duolingo and jump ahead a few units (easy to do), then find a class when you're more certain about where your ability is.

That's an option, although I am mainly interested in understanding people talking.

I’ve experienced this exact problem, it’s super frustrating! My first language is English, I learned German in school and university, then learned Swedish later in adulthood. As a result I have a decent grasp of Norwegian when it comes to reading and (to a lesser extent) listening, but I really want to improve my speaking and writing. Yet it seems super tricky to do so. I actually took a course called Norwegian for Swedish speakers, and while it was interesting, it wasn’t very useful sadly. I guess that either doing boring old school vocab learning, or alternatively just gradually improving by exposing oneself to the language is the answer. Interesting situation to be in tho, for sure.

Find videos of the news in your target language on YouTube, so you can understand what happened and then when they interview people about what happened. The game show one is also good. I found I could understand Italian quite well (I speak Romanian) from watching some bizarre game show called something like Tutti Frutti.

I would watch TV in that language, read websites, magazines, etc. Just expose yourself to as much as possible. At the start you won't understand much, after a while you will. You should be able to understand people from that. If you want to also be able to speak and write, then you will need more active engagement.

I am doing Catalan Duolingo. So much is nearly identical to Spanish and I love getting the praise and air punching avatars when I translate something that is just a few letters different!!!

Have you thought of trying AI? I've been learning Spanish on and off for years but would really like to step it up a level. Problem is my vocab and grammar and comprehension are way ahead of my spoken language. I joined an intermediate class in a lang school this year but the lessons are boring, yet I can't move up a level because I get tongue-tied.

So, I looked at the AIs being advertised - Jumpspeak, langua, TalkPal... In the end, I opted for TalkPal and am half way through my free trial. After that it's 15/month I think (cheaper than my in-person lessons by far). In just a week, doing about an hour a day, I can feel the improvement. I'm gonna pay for at least a month and then decide what's next. I just hope cancelling will be easy when it comes to it.

I did this. I speak fluent French and German (I teach them both). I decided to learn Spanish. Because of my French, I found it very easy and didn't need to go to classes. I learned almost entirely by listening. I started with beginner/intermediate level podcasts and then started listening to audiobooks on Audible, slowing them down so they were easier to understand. I started with Spanish translations of English books I already knew really well (like the whole Harry Potter series!) and eventually moved on to Spanish audiobook novels. It's a brilliant and fun way to learn! I ended up studying for an A Level and now teach KS3 Spanish.

When I did Spanish many years ago at school we started it at 14 but French from 11. The people who were good at French had the option to do Spanish GCSE in 2 years.
Our Spanish teacher was also a French teacher and she often taught us from French to Spanish with no English involved. It was a really good way of learning and even now sometimes when I am translating from Spanish I think of the French not English word.
I am currently learning Italian and Portuguese on DuoLingo and find it easier than say German that I was doing more or less from scratch. I did do Latin at school for a couple of years too that I think helps

I learnt Spanish in the same way actually, but I'm not prepared to do that for this new language. In a two-year GCSE you're still learning every word and grammatical structure from scratch, which I don't find necessary when my aim is mainly just to understand and not speak/write myself.

Thanks. Interesting that you mention slowing down an audiobook. I hadn't really realised I could do that and I've been looking for podcasts that are deliberately slow for learners. It's another option to look at.