Yeah, that's a holdover from the original original game from the 90s where a big part of the early game was simply struggling to detect alien activity to stop them in the firat place while they stealthily tried to infiltrate the world governments to cut your limited funding.

You weren't the best and last line of defense, you were just the people crazy enough to believe aliens were a threat, with tenuous connections to get funds from governments to try and fund the fight. There's plenty of in game data about how all your starting equipment is military hand me downs and whatever mothballed prototype designs your team could snatch up on the cheap and retrofit. You can also field squads of up to 14 soldiers from the start, which helps because things are far more of a meat grinder and less of a "just keep enough squads to rotate folks out". Up through mid game, losing two soldiers a mission isn't unusual.

The initial scrappiness and meat grinder aspects were significantly lessened in the more recent XCOM reboot from 2012, which tends to be a more fun experience overall, but does lead to a little bit of dissonance.

Should I install those 2 DLC before starting the game or should I go in vanilla and get the DLC later?

I'd also add that you shouldn't ignore mods if you wanna get the most out of the game. First playthrough, yeah go without mods and get a sense of the game. But I have like an embarrassing amount of time in this game and I think I did one or two playthroughs without mods, so a majority of my playtime is thanks to mods, especially long War of the chosen.

(well, in terms of polish and mod support).

I misread this as "Polish mod support" and thought that was oddly specific, but I'm always happy to get new language mods for the game, frustrating when an international team all speak the same US english or British english.