I think the answer to your question depends on the type of document (I’m not familiar with a Translation Plan). If this is a general plan that applies across all of your clinical trials, then perhaps it should be treated in the same way as your other process documents (SOPs, WIs etc). These are generally not filed in the TMF but rather, a list of procedures is included (artifact 01.01.04).
However, if the plan varies from trial to trial, then I think there are two primary options open to you. It could be considered as filling the definition of a TMF Plan 01.01.01 (“To describe how records for the trial will be managed and stored during and after the trial….”) or within the scope of artifact 01.01.05 Operational Procedure Manual (“To describe trial-related processes not covered by formal standard operating procedures…”).
I don’t believe I’ve ever come across a Translation Plan being written as a stand-alone document, hence my hesitation in being more definitive in my response! Translation requirements/processes are more typically described in other documents. The TMF Plan or a TMF SOP then describes how translations, back translations and translation certificates are managed in the TMF. Hope this helps.