Hi, here are some answers and comments to your comments and questions:
The community of Tackler is next to non-existing, that's true. However Tackler is actually used based on OSS-repository statistics and stats are increasing over time. So there is some traction of it e.g. e257/accounting/tackler#2
Tackler core itself is actually really small, few thousand lines of code, so with all the tests it should be fairly easy to take over it. Also used Scala code is pretty straightforward. These should provide some insurance against small community.
It would be ok-ish to have some java code with Tackler, if reason is good and well justified. One example would be in-memory Git repository writer: GitDB On the other hand, Scala is really nice language and it's possible to move from Java to Scala in gentle way (I did that - Tackler isn't very die-hard Scala).
If you could point worst places where documentation is lacking or provide patches, that would be really awesome.
Easy requirements for contributions are that everything is signed-off (e.g. every single commit) and it would be really nice if you could also PGP sign commits. Please see CONTRIBUTING, it has some information.
Then the more difficult and vague stuff about contributing:
There are some reason why tackler exists in first place and some high level goals in README. At the moment aim of Tackler is to have really narrow focus, but that minimal feature set should be done extremely well and trustworthy.
Also all features must be tested really well. This is unbreakable rule for Tackler. I won't accept any changes (my own or someone's else) if it degrades seriously test coverage. There might be exceptions if it's truly too difficult to test reasonably, but it should be exceptional situation.
Tackler's idea is to be minimal core and cli interface is "reference implementation" of those features of core, but not necessesary full CLI UI. For example, I have several applications where all kind of business domain specific filters are defined in shell script and fed to tackler via --api-filter-def as base64 armored ascii strings. But those filter combinations are not implemented as CLI options for above reason.
On the other hand, if the idea is cool, then by all mean let's have it in the Tackler (e.g. accounting checksums and geo location).
My plans for tackler are: It will be maintained and extended as needed. (ATM I have tag support on the pipeline). Tackler's README has some high level ideas for future. I typically work with Tackler during nothern hemisphere winter and not so much during summer. However, this winter has been exception. Tackler probably won't ever have as many options and features as ledger, hledger or beancount. And it won't have ledger-compatible format.