Geolocation might be worth considering. Just browse lemmy from that instance before creating an account and see if it's smooth, in terms of latency.
Language and subject would be of no concern for me. You can subscribe to your communities of interest on any other instance (barring defederation). Some instances even have no communities on their own. There is a slight advantage, you could use the 'Local' tab. I just use 'Subscribed'.
Server administration and federation are probably the most impactful factors.
For administration, read the instance sidebar and check the local community (often called 'meta'). If the admin posts info and updates there, and you like their style, that's great. What you probably want to avoid is an unreliable instance which suddenly goes out of business.
For federation, check the /instances tab of the instance in question. You can also check https://fba.ryona.agency/?domain=lemmy.world for a reverse search ("what other instances block my instance?"). Note, when 'Last seen' is an old date, that probably means the block was lifted.
You can put lots of time and effort into this, but you don't need to. Just go with whatever feels right for you (in terms of instance choice, but also in terms of time and effort invested). You'll most probably be fine with a lucky pick.
It's also possible to have and use several accounts on several instances.