check if translation exist before adding alternate tags

[timnarr]

Hi Tobias,

thanks for this plugin 🙌

I just noticed that kirby-seo outputs alternate tags for pages that doesn't have a existing content file for this language.

I've added a simple check by using the page methods translation() and exists() to check if the page for a given language code exists on disk.

Pls let me know if you need more information from me.

[leobardimpact]

Thank you for this approach. I used it and extended it for this pull request: https://github.com/tobimori/kirby-seo/pull/128

[dweidner]

Just noticed the same problem. Wanted to suggest a similar fix.

[tobimori]

will be included manually in v2.0.0 - thanks for your PR.