Most of the Warlock Improved Familiars can turn themselves Invisible - The Quasit, Sprite & Imp can, the Pseudodragon cannot. It just depends on what other benefits you want.
- Pseudodragon: +3 Perception (and Advantage), +4 Stealth, Blindsight 10 ft & Darkvision 60 ft, understands Draconic, Telepathic, can sting a target to give the poisoned condition, and potentially knock targets unconscious. Fly speed 60
- Quasit: +5 Stealth, Darkvision 120 ft, Understands Abyssal, can Scare a target 1x/day, and attacks can give the poisoned condition. Land speed 40
- Sprite: +3 Perception, +8 Stealth, understands Elven & Sylvan, can magically know the emotional state of creatures touched & potentially alignment, can shoot a target at 40/60 to give the poisoned condition, and potentially knock targets unconscious. Fly speed 40
- Imp: +4 Deception, +3 Insight, +4 persuasion, +5 stealth, darkvision 120 with devil's sight, understands Infernal, and attacks deal decent damage (1d4+3 piercing and an additional 3d6 poison on a failed Con Save, or save for half). Fly Speed of 40
What this means is you can:
- "translate" those languages by using an action to move your senses into your familiar
- Forgo your attacks to have your familiar attack as a reaction, or channel a touch spell through the familiar to make it ranged
- Telepathically communicate to have your familiar use its own turn to perform a non-attack action (which would not break invisibility), including Search, Help, the Quasit's Scare, or the Sprite's Heart Sight, on the familiar's turn
Note, you can always change your familiar out to a different one via ritual. So if you normally have an invisible imp on your shoulder (for example, because your Wisdom(Insight) is garbage and you don't have darkvision), but know everything in this area you're entering only speaks Giant, Aquan, or some other exotic language no one in your party knows, you can spend an hour switching to a Pseudodragon to be able to communicate telepathically simple ideas, emotions or images.