[RPG] Is it normal that players share their characters’ thoughts

Some people like to "act" in RPGs, some like to tell stories, and some want to roll dice. Most players I know expect all three activities, but in different amounts. It is unlikely you will ever be in a group where the balance is the same for each player and the DM.

In my opinion you have already done the right things by not pushing too hard, and having a chat to see if there is any problem. I think to follow that, you should accept the player for who they are, and make the best game you can with how they currently contribute.

Where you can, take a "yes" or a "no" from the player, and treat it as if the PC had said something a little more in character. Don't have the NPCs react to the lack of social niceties, but respond instead as if the PC controlled by this player had given the essence of what they said, but more appropriate to the situation. If you want, cover that with a little description (but avoid temptation to voice the request as the PC, narrate it instead).

If the player has a character with good social skills on the character sheet, and the system allows for it, let the player state intent without acting in character, but narrating desired outcome: "The fighter explains that he is a veteran of several wars, and worth more money than the merchant is offering. I want to persuade the merchant to offer more for the quest."

Continue to give them opportunities to contribute further, but don't make a big deal of it.

It sounds like your players are engaging with the NPCs, they're just not asking the questions you want so they can get the information you've prepared that will help them. Doesn't sound like disinterest so much as they interact differently than you would, or than you'd expect. It's unlikely that you'll be able to change the way they act, at least not directly, so you're probably going to have to change your style a bit to correct the mismatch.

In general terms, you may have the classic problem that often arises in mystery RPGs; the players miss the clues, don't ask the obvious questions, and otherwise fail to extract the appropriate information from the scene to move the plot forward. It's a common disconnect, usually due to the fact that the GM has a lot more information, so connections that seem obvious are actually much less so to the players. To counteract this difference, you'll need to put a bit more window dressing around the points that they need to discover that are critical to the plot.

I recommend reading Justin Alexander's essays on the Three Clue Rule. It gives some great advice on how to improve the players chances to ferret out what they need from the setting without just handing them the info. Ultimately, if there's a piece of information that was important enough that you've actively called it in your setting notes, you're going to need a couple of back-up opportunities for the players to discover it, because they're going to miss the first two glaringly obvious opportunities to do so.

A last tidbit that I find works for some groups is profiling,, i.e. put stuff where they look. Pay attention to the details the players engage with and try to get a feel for their patterns. Maybe they tend to engage with shopkeepers but not people on the street. Maybe the wizard is always looking for cool wands. If you want them to ask the locals for more info on the ants, put a cool wand proudly displayed in the local shop. The shopkeeper will tell them it's not for sale, but he'll ramble on a bit about the wizard that created it, and how he lived here to study the physiology and tactics of the ants. As one of three clues that they can get more info to help in their fight, it starts to give you a chance to get the message across.