Unable to use "da" as a tag name

I'm unable to use `da` as a tag name. When I try to apply it, I get the following:

```
$ timew tag @1 da
At least one tag must be specified. See 'timew help tag'.
```

I use Timewarrior for tracking language learning progress, and `da` is a language code for Danish. I'm able do use other such language codes as tags without an issue: `de` for German, `nl` for Dutch etc.

OS: Debian Trixie (13.4)

Version of Timewarrior: tested with 1.9.1 release. I tried compiling the latest development commit, but I ran into an [issue](https://github.com/GothenburgBitFactory/timewarrior/discussions/754).

Start by reproducing the issue with `timew tag @1 da` and compare it with the working `de` and `nl` tags. Read the implementation or entry point for the `tag` command and its argument handling; done means `da` is accepted as a tag while the existing tag validation remains intact.

Written by the indexing model from the issue text.