If you’re past 30 you’ve probably been programming for at least 8 years. If you’ve never touched another language...yes...that is a concern.
How old are you? I'm closer to 40 than I am 30 and I don't put projects I worked on when I was 22 on my resume. If I can't remember the details, it's not going on my resume. If I can't give good/interesting answers when asked about that project, its not going on my resume. I don't want to fill my resume with a lot of irrelevant crap since I have real experience to put on my resume.
I've worked with many languages both professionally and casually, probably over a dozen over the years. I only put the ones I have mastered on my resume.
If I were conducting an interview for a mid-level position (and I do) and I asked a candidate about their experience in Rust, which is on their resume, and it's anything but over a year of professional or semi-professional development, then I'm going to assume the rest of their resume is just as bogus. A weekend hack-a-thon doesn't "count" as "experience."
As someone who actually has a say in hiring decisions, I'm not going to think highly of a candidate with over 5 years of experience who puts code they only spent the better part of a weekend on on their resume. That means they don't understand what experience actually is.
> I've worked with many languages both professionally and (very) casually, probably over a dozen. I only put the ones I have mastered on my resume.
And that is excellent and I would consider that perfectly normal and rational behavior. You have more than one. You took the time to explore. It's a field of constant problem solving and lack of exploration of solutions is something that sticks out as a giant red flag because of that.
More experience is definitely better. If I were hiring for a particular language I'd certainly want more than a weekend's experience, but some experience is still better than no experience.
At my last job we interviewed a guy who was about 5 years out of college and had been working in a .NET shop the entire time. He was applying for a ruby position and his only experience was spending a couple of weeks in his spare time building a Rails site. From that minimal experience, we were able to get him to explain why he decided to pursue it, what he liked about it, what problems he was trying to solve, what weaknesses he was trying to overcome that he was seeing in .NET, etc.
Despite the lack of language experience, we hired him based on the "geek gene" that clearly showed problem solving skills and desire to learn. The guy was absolutely rock solid and became one of our most valuable team members within about 2 months time. I believe he's the CTO of a funded startup now if I remember correctly.
If he'd had only Rails and I'd asked him about what weaknesses he saw with the stack...my very next question would have been what he tried to do to overcome those weaknesses. Maybe he answers about what other things he tried within the stack, caching techniques, using jRuby, refactoring in certain ways, etc and as long as that could be articulated it would generally be okay. In general though, seeing how other languages handle the same problem is going to be perfectly normal part of the process...which should lead to having more than one language on the resume.