And if we can read/write we can speak it right?
Negative. Reading and writing does not confer the ability to speak a language coherently. If you know a written language, you can translate it to a language you do know, but you will make errors in pronunciation, context or conjugation when you try and speak it.
Imagine someone speaking French with a very anglicized accent ("merci garçon" spoken as "murky garkon"), or someone for whom English is a second language (like an outsourced helpdesk). To a native speaker, they may be nearly unintelligible.
So, all the PC's speak Cymric. The Northmen also speak Nord, and our Irish knight speaks Irish. The outlanders speaking Cymric was for story purposes, and handled in their back-story. If any of you wish to learn other languages, you will need to spend time, points and resources to learn. Such expenditures would count towards "Conspicuous Consumption" which would earn you Glory.
Significant efforts in this regard might earn you an additional title, such as "Sir Cedwyn the Learned."
I'm more specifically asking about/interested in German if Rodric decides to go after his father's sword. I believe it was in Saxon lands.
Saxons speak Sais (or Saxon), one of the western-Germanic family of languages, which themselves descend from the Teutonic language root. This is similar to how Irish and Scots are both members of the Celtic family of languages, but are distinct. German does not exist as a distinct language in this period.
Rodric could certainly hire a tutor to teach him the Saxon tongue during Winter Phase, and this would count as Conspicuous Consumption. How proficient he becomes in that language depends on the skill points and advancement devoted to it.