Identify which text is missing
A poster title rendered as boxes and missing characters in a subtitle need not use the same font source. Before installing a font anywhere, identify whether the text belongs to a generated cover image, a burned-in subtitle or a subtitle rendered by the current client. Keep the title, subtitle format and device with the observation.
Apply Jellyfin’s font-source explanation
Jellyfin says server system fonts are used for subtitle burn-in and generated cover images. Client system fonts supply interface text and some subtitle rendering. Its fallback-font folder is used by the web client for ASS characters when other fonts cannot provide the glyphs. The fallback set has a total-size limit and is preloaded by the browser, so a large indiscriminate font collection is not the documented solution to every missing character.
Compare a specific language sample
For example, if Japanese subtitle glyphs are missing only in the browser while another supported client renders the same track, record that difference before changing the server’s whole font environment. Check whether the media contains font attachments and which rendering path is active. Use an appropriately licensed font covering the needed characters in the documented location, then revisit the same passage. Do not confuse a correct font with a missing translation or a different subtitle edition. The meaningful result is whether the required characters render in the intended path without replacing the viewer’s chosen track.
Questions, answered
Will installing a font on the server necessarily fix every client-rendered subtitle?
No. Jellyfin documents different font sources according to how the text is rendered.