You can run Weglot and Plug in SEO Plus without issues. However do note that due to the way Weglot handles translations you should consider a few things when performing SEO on your shop.
Weglot works by looking at an exact string of text in your shop. For example 'coffeebeans from Kenya'. Then it will create an automatic machine translation which can be overridden by you entering a manual translation like 'kaffebønner fra Kenya'.
If you have spent a lot of time translating all of these phrases manually be aware that even a tiny change to the string of text will 'lose' the manual translation.
For example if you change 'coffeebeans from Kenya' to 'coffee beans from Kenya' (adding a space) then the manual translation you entered will no longer be used, switching to an automatic machine translation.
Since the automatic machine translation could be poorer or even incorrect vs. your manual translation, it is an important consideration when changing content on your shop.
This content includes your SEO elements like page titles, meta descriptions, headings, ALT tags and other aspects covered by Plug in SEO recommendations. When changing these things be mindful of the potential impact to any manual translations you've entered in Weglot.
Our recommended approach is to perform your whole-site SEO actions like entering templates first. Then enter any manual translations you wish in Weglot.
Get in touch if you have any questions or concerns about using Plug in SEO Plus with Weglot. Together they are a strong multilingual SEO solution for Shopify.