There are several apps available for the Frisian language. Frisan (Frysk) is also included in Google Translate. An overview of apps and tools you can find on Taalhelp.frl (not available in English).
Both Gboard and Swiftkey included Frisian into their keyboard apps, to help you to write in Frisian more easily.
This English-Western Frisian Dictionary and Western Frisian-English Dictionary-app does also work offline:
Frisian-English Dictionary app
This book teaches you the grammatical rules of Frisian in a joyable and effective way: Frisan Grammar – the basics