I can't decide which languages to choose.

My character is a wizard (with sage as background) who is obsessed with dragons and knowing things in general. They are a half elf.

The languages I currently have are Common, Elvish and Draconic, but I can choose two more.

Depends on the campaign setting and the rest of the party, really. If you know/expect you'll be dealing with Fey, Fiends, etc. then it can't hurt to take a corresponding language, but languages are too soft a feature to really allow for objective recommendations.

Ask your DM, and party they will be the most help.

Primordial, Undercommon, and Deep Speech are probably the three with the widest variety of things they'll let you talk to, but nearly everything either speaks Common, has telepathy, or has no interest in communication.

By default; languages are one of the things DMs can very freely shuffle around on a stat block, since they have no impact on balance.

i'm kind of looking for thematic suggestions, not just ones that would be useful, but thanks for the suggestions anyway

Thematic is largely based on the setting, character, and plot, and from what info we have you already hit the one option that stands out.

I choose Reason
As a wizard and sage, understanding the language of celestial beings can give access to divine knowledge and ancient texts. This language is also useful for communicating with celestial creatures and understanding holy scriptures.

I like primordial because you can also speak the (elemental) sub languages. Other than that, it depends on what might be in your campaign. I sometimes take sylvan which could make sense for a half elf.