there is an alternative, RH Voice for text to speech and transcribro for voice input. Try those first.
....admittedly, Google's implementation is far superior.
I do consider the app a privacy concern due to its microphone access that can be accessed at all times unless you disable it. But it really depends on your threat model and how anti-google you are. There are GrapheneOS users that have it installed without a care in the world.
I believe due to IPC, apps on the same profile can communicate with each other. From what I've interpreted, this would mean that even if you deny network access to Google Speech Recognition, it may still communicate with Google's servers if you have another Google app installed on that profile with network access, which includes Sandboxed Google Play Services.
Fortunately, Google Speech is one of a handful Google apps that doesn't require network access or Sandboxed Play Services installed to work. However, you will need to grant it temporary network access to initially download the language/voice models within the app and then disable network afterwards. I would recommend doing this on a VPN if you want to try to hide from Google.
I'm not sure if you need sandboxed Google play in order to do the initial languages download, so try it without it first. You can uninstall play services afterward.
Do this on a profile that won't need Google play services or other network access Google apps afterwards, creating a new profile for this purpose if necessary.
...though, your language learning apps themselves may need play services installed in order to function and at that point, there's really nothing you can do except keep it on a separate profile with an always-on VPN. To save battery life and prevent these Google apps from pinging Google's servers when not in use, turn off that profile when not in use.